In current GWT svn (as of at least 18 Jul 07), the constructor for com.google.gwt.core.client.JavaScriptObject has been changed from taking a single int parameter to taking no parameters.
This results in a bunch of errors when starting up gwt in hosted mode, similar to the following one:
[ERROR] Errors in 'jar:file:/home/[omitted]/WebContent/WEB-INF/lib/gwt-widgets-0.1.3.jar!/org/gwtwidgets/client/ui/gsearch/GResult.java'
[ERROR] Line 26: The constructor JavaScriptObject(int) is undefined
I'm running jdk6u2 on FC6