Also, most frameworks are evolving FAR quicker than GWT is.
Thirdly, GWT will still require server-side coding. You might as well use a framework with an Ajax library or just plain html output, and then add-in GWT or Adobe Flex later, if at all.
Fourth, GWT does not have anywhere near the community or IRC users that most frameworks do.
But, mainly, GWT is a bad idea because it loses the advantage that web frameworks give to web startups over desktop development. That's because using GWT can be even slower than using traditional desktop development tools like Visual Studio because you not only have to compile, but also upload the files to get a real idea of what you're doing.
Why not just use a console with a web framework, or something with both in one like Seaside? As soon as you save the file, the change exists.