First off, let's start of with some good links for testing with GWT:
This is one of my new favorites, Rob describes approaches for developing UI's in GWT that are test driven. This uses a great approach of the MVC pattern paired with the Humble Dialog approach. This allows one to test most of the view code with lightening-fast JUnit (or any other xUnit) tests. Used appropriately, this is a huge advance in WebApp testing and productivity.
http://robvanmaris.jteam.nl/2008/03/09/test-driven-development-for-gwt-ui-code/
And a shorter GWT+MVC primer post by Iwein Fuld:
http://blog.springsource.com/main/2008/02/19/enabling-test-driven-development-in-gwt-client-code/
More to come!
