I have just spotted the Cross site tutorial on the GWT docs.
http://code.google.com/webtoolkit/doc/latest/tutorial/Xsite.html
Within that document it shows a way of getting data from a remote
sever using native JSONP methods. It might be worth updating the
documentation to show developers how to use the JsonpRequestBuilder
object and make calls that way?
Just a thought
Eggsy