In the last several days several of us have been forced to update to a beta version of Eclipse 3.X.
It seems SourceForge has applied a CVS security patch that interacts Poorly with Eclipse 2.1.1.
The problem shows up as the top level folder under the HEAD tag cannot be checked out. A workaround it is to check out a branch and the "replace" the branch with the contents of HEAD.
Or upgrade to a beta version of Eclipse 3.X.
Cheers, hope this saves any Eclipse users some confusion when working on geotools2/geoserver.
Jody