Comment by ltearno: This is too bad (at the end of the document) :
The implementation of overlay types in GWT 1.5 does not support Generators defining new JSO subtypes (due to the need to redefine or otherwise extend JavaScriptObject$ during subsequent compilation). This restriction is still in place.
Being able to generate sub classes of JavaScriptObject would be so coool For more information: http://code.google.com/p/google-web-toolkit/wiki/OverlayTypes -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
