Hi,
You might check out the interview I did with InfoQ. It includes a sample chapter from my book and it happens to be on SEO.
It's not trivial, but I think the solutions in the chapter let GWT work nicely in an environment where SEO is important. The basic solution is to implement something I call 'bootstrapping'. This means that your pages take the info that would normally come across in GWT-RPC requests and serialize them into the page. The GWT widget then loads this information without an RPC request. While your page serializes the info into JavaScript, it's easy to also write a <noscript> to the page that can be used for SEO.
Take a look at the PDF included here: InfoQ GWT it goes into all the detail. The whole sample project is here: google code with source on github.