After an interesting JavaOne session on GWT, I thought I would have a quick play with the toolkit. I hit a little roadblock initially which after discussions with other colleagues they had problems with too.
In the Getting Started guide the section on building an Eclipse project skips one step that caught me out. Do not create the project in your Eclipse workspace, instead perform the two steps in a temporary area. Then import that temporary project using Eclipse/import/Existing files into workspace - be sure to click the copy source.
I found if I didn't do that I get errors about the project I was trying to import being already inside the workspace.