If you create a project with maven using the maven-archetype you will get a part in your pom (probably commented out) where you see how you can recompile your widgetset. If you want to do it manually in eclipse (for development purposes) just activate the vaadin-plugin in your project facets (you must have the vaadin plugin installed in eclipse).
if I uncomment the part of the pom for widget compilation all I get is a message like “there is no new gwt widget to compile” (I am not on my workstation now)
vaadin plugin if I press ctrl+6 tells me to select a gwt.xml or a vaadin project to compile.