This patch creates gwt-servlet-deps.jar, which includes validation.jar and json.jar for now. WebAppCreator includes gwt-servlet-deps.jar in the libs/ folder and on the classpath unless you specify -maven, in which case it does not include them.
http://gwt-code-reviews.appspot.com/1047802/show -- http://groups.google.com/group/Google-Web-Toolkit-Contributors