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I suspect that the problem is not in the Vaadin build but in the version of Vaadin plug-in for Eclipse that you are using.
Try to upgrade the plug-in from the update site http://vaadin.com/eclipse/experimental . It also contains some other fixes and improvements for Vaadin 7.
After upgrading, if necessary, change the version of Vaadin in the project to 6.8.0 (via Project properties -> Vaadin) and then back to Vaadin 7 alpha 3 (optionally after manually removing GWT dependencies from the build path). Note that the current version of the plug-in might not add the validation JARs on the build path - but they are not strictly necessary even though GWT compiler will issue some "error" messages.