Exception occurred to me after upgrading and using the eclipse helios with gwt 2.1 new plugin.
Don't know if both connected or not or maybe it's subversion problem.
I have looked and googled for solutions but nothing yet over the web.
What's helped me:
Closing all the projects in eclipse.
Opening all the projects.
This probably synchronize eclipse with the file system. Then Project->Clean all.
That's it, worked for me, hoped that it will help you also.