The goal of sharing a JSON lib across client-server seems great to me. I think super-source is the way to really make that go.
However, for present purposes this is for gwt-dev to use, so I think it's fine landing this without respect to the future uber user lib we want to build. We could always update the dev code in the future to use the uber user lib. -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
