Afters struggling for almost 3 days, I finally got some vaadin add-ons working for me.
No matter what I did, I could not get maven to compile the custom widget sets :'-(
Wanting to do it with maven I started off with the instructions at 
http://dev.vaadin.com/wiki/Articles/MavenIntegration.
I got error messages like:
Failed to load the widgetset: 
/my-project/VAADIN/widgetsets/my.company.my.project.widgetset.My_ProjectWidgetset/
my.company.my.project.widgetset.My_ProjectWidgetset.nocache.js?1270654337749
Building it with the eclipse plugin said: 
Compiling widgetset my.company.my.project.widgetset.My_ProjectWidgetset
Updating GWT module description file...
07 Apr 2010 5:46:09 PM com.vaadin.terminal.gwt.widgetsetutils.ClassPathExplorer getAvailableWidgetSets
INFO: Widgetsets found from classpath:
 com.vaadin.terminal.gwt.DefaultWidgetSet in jar:file:/stf/opt/maven/repository/com/vaadin/vaadin/6.2.6/vaadin-6.2.6.jar!/
 my.company.my.project.widgetset.My_ProjectWidgetset in file:/stf/prj/My_Project/src/main/resources
Done.
Starting GWT compiler
Widgetset compilation completed
But when running the webapp, I got:
Widgetset does not contain implementation for org.vaadin.browsercookies.BrowserCookies. Check its @ClientWidget mapping, widgetsets GWT module descrioption file and re-compile your widgetset. Unrendered UIDL:
 
org.vaadin.browsercookies.BrowserCookies(NO CLIENT IMPLEMENTATION FOUND) id=PID7
 
variables
Widgetset does not contain implementation for com.github.wolfie.meteorcursor.MeteorCursor. Check its @ClientWidget mapping, widgetsets GWT module descrioption file and re-compile your widgetset. Unrendered UIDL:
 
com.github.wolfie.meteorcursor.MeteorCursor(NO CLIENT IMPLEMENTATION FOUND) id=PID8 gr=75 th=10 pl=1000 di=2 im=theme://images/logo.png fr=0 disabled=false
I thought that it was building, since the addons that I wanted to use got detected.
Little did I know that it was not actually building!
Building with maven said:
[INFO] Updating widgetset my.company.my.project.widgetset.My_ProjectWidgetset
[INFO] establishing classpath list (scope = compile)
[ERROR] 07 Apr 2010 5:38:31 PM com.vaadin.terminal.gwt.widgetsetutils.ClassPathExplorer getAvailableWidgetSets
[ERROR] INFO: Widgetsets found from classpath:
[ERROR]  com.github.wolfie.meteorcursor.MeteorcursorWidgetset in jar:file:/stf/opt/maven/repository/com/github/wolfie/meteorcursor/1.0.0/meteorcursor-1.0.0.jar!/
[ERROR]  org.vaadin.browsercookies.widgetset.BrowserCookiesApplicationWidgetset in jar:file:/stf/opt/maven/repository/com/vaadin/sami/browsercookies/1.0.2/browsercookies-1.0.2.jar!/
[ERROR]  com.vaadin.terminal.gwt.DefaultWidgetSet in jar:file:/stf/opt/maven/repository/com/vaadin/vaadin/6.2.6/vaadin-6.2.6.jar!/
[ERROR]  my.company.my.project.widgetset.My_ProjectWidgetset in file:/stf/prj/My_Project/src/main/resources
[ERROR] 
The reason it was not building was because I was trying to use the stable version of the gwt-maven-plugin: 1.2
After reading this forum post again, I tried 1.3-SNAPSHOT of this plugin; and all my troubles went away.So now I build it with maven, and it should give output like this:
[INFO] [gwt:compile {execution: default}]                                                                                                                    
[INFO] using GWT jars from project dependencies : 1.7.1                                                                                                      
[INFO] auto discovered modules [za.co.enerweb.ebr.gui.vaadin.widgetset.Ebr_vaadin_webWidgetset]                                                              
[INFO] establishing classpath list (scope = compile)                                                                                                         
[INFO] establishing classpath list (scope = compile)                                                                                                         
[INFO] establishing classpath list (scope = compile)                                                                                                         
[INFO] establishing classpath list (scope = compile)                                                                                                         
[INFO] establishing classpath list (scope = compile)                                                                                                         
[INFO] establishing classpath list (scope = compile)                                                                                                         
[INFO] Compiling module my.company.my.project.widgetset.My_ProjectWidgetset                                                                       
[INFO]    Scanning for additional dependencies: jar:file:/stf/opt/maven/repository/com/vaadin/vaadin/6.2.6/vaadin-6.2.6.jar!/com/vaadin/terminal/gwt/client/DefaultWidgetSet.java                                                                                                                                           
[INFO]       Computing all possible rebind results for 'com.vaadin.terminal.gwt.client.WidgetMap'                                                             
[INFO]          Rebinding com.vaadin.terminal.gwt.client.WidgetMap                                                                                            
[INFO]             Invoking                                                 
[INFO]                Detecting Vaadin components in classpath to generate WidgetMapImpl.java ...                                                             
[INFO]                Widget set will contain implementations for following components:                                                                       
[INFO]                  com.github.wolfie.meteorcursor.MeteorCursor                                                                                           
[INFO]                  com.vaadin.ui.AbsoluteLayout                                                                                                          
[INFO]                  com.vaadin.ui.Accordion                                                                                                               
[INFO]                  com.vaadin.ui.Button                                                                                                                  
[INFO]                  com.vaadin.ui.CheckBox                                                                                                                
[INFO]                  com.vaadin.ui.ComboBox                                                                                                                
[INFO]                  com.vaadin.ui.CssLayout                                                                                                               
[INFO]                  com.vaadin.ui.CustomComponent                                                                                                         
[INFO]                  com.vaadin.ui.CustomLayout                                                                                                            
[INFO]                  com.vaadin.ui.DateField                                                                                                               
[INFO]                  com.vaadin.ui.Embedded                                                                                                                
[INFO]                  com.vaadin.ui.Form                                                                                                                    
[INFO]                  com.vaadin.ui.FormLayout                                                                                                              
[INFO]                  com.vaadin.ui.GridLayout                                                                                                              
[INFO]                  com.vaadin.ui.HorizontalLayout                                                                                                        
[INFO]                  com.vaadin.ui.Label                                                                                                                   
[INFO]                  com.vaadin.ui.Link                                                                                                                    
[INFO]                  com.vaadin.ui.ListSelect                                                                                                              
[INFO]                  com.vaadin.ui.MenuBar                                                                                                                 
[INFO]                  com.vaadin.ui.NativeButton                                                                                                            
[INFO]                  com.vaadin.ui.NativeSelect                                                                                                            
[INFO]                  com.vaadin.ui.OptionGroup                                                                                                             
[INFO]                  com.vaadin.ui.OrderedLayout                                                                                                           
[INFO]                  com.vaadin.ui.Panel                                                                                                                   
[INFO]                  com.vaadin.ui.PopupView                                                                                                               
[INFO]                  com.vaadin.ui.ProgressIndicator                                                                                                       
[INFO]                  com.vaadin.ui.RichTextArea                                                                                                            
[INFO]                  com.vaadin.ui.Select                                                                                                                  
[INFO]                  com.vaadin.ui.Slider                                                                                                                  
[INFO]                  com.vaadin.ui.SplitPanel                                                                                                              
[INFO]                  com.vaadin.ui.TabSheet                                                                                                                
[INFO]                  com.vaadin.ui.Table                                                                                                                   
[INFO]                  com.vaadin.ui.TextField                                                                                                               
[INFO]                  com.vaadin.ui.Tree                                                                                                                    
[INFO]                  com.vaadin.ui.TwinColSelect                                                                                                           
[INFO]                  com.vaadin.ui.Upload                                                                                                                  
[INFO]                  com.vaadin.ui.UriFragmentUtility                                                                                                      
[INFO]                  com.vaadin.ui.VerticalLayout                                                                                                          
[INFO]                  com.vaadin.ui.Window                                                                                                                  
[INFO]                  org.vaadin.browsercookies.BrowserCookies                                                                                              
[INFO]                Done. (2seconds)                                                                                                                        
[INFO]    Compiling 1 permutations                                                                                                                            
[INFO]       Permutation compile succeeded                                                                                                                    
[INFO]    Linking into /stf/prj/My_Project/src/main/webapp/VAADIN/widgetsets                                                                          
[INFO]       Link succeeded                                                                                                                                   
[INFO]    Compilation succeeded -- 40.785s
To be able to serve my webapp using tomcat launched from eclipse I need to compile the GWT stuff into src/main/webapp . So unlike the instructions I mention at the top of this post, some key parts of my pom is as follows:
  <properties>
    <vaadin-widgets-dir>src/main/webapp/VAADIN/widgetsets</vaadin-widgets-dir>
  </properties>
  <build>
    <plugins>
      <!-- Compiles your custom GWT components with the GWT compiler -->
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>gwt-maven-plugin</artifactId>
        <version>1.3-SNAPSHOT</version>
        <executions>
          <execution>
            <configuration>
              <webappDirectory>${vaadin-widgets-dir}</webappDirectory>
              <extraJvmArgs> -Dgwt.style=PRETTY -Xmx512M -Xss1024k</extraJvmArgs>
            </configuration>
            <goals>
              <goal>resources</goal>
              <goal>compile</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
      <!-- Updates Vaadin 6.2 widgetset definitions based on project dependencies -->
      <plugin>
        <groupId>com.vaadin</groupId>
        <artifactId>vaadin-maven-plugin</artifactId>
        <version>1.0.0</version>
        <executions>
          <execution>
            <goals>
              <goal>update-widgetset</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
      <plugin>
        <artifactId>maven-clean-plugin</artifactId>
        <configuration>
          <filesets>
            <fileset>
              <!-- also delete the generated vaadin web content -->
              <directory>${vaadin-widgets-dir}</directory>
            </fileset>
          </filesets>
        </configuration>
      </plugin>
    </plugins>
  </build>