We've seen some reports of client connectivity problems when people upgrade a previous version of TFS to TFS 2010 and configure it to you the same URL.  This is the result of some stale client cache info.  Bryan Krieger has written a post on recognizing and fixing the issues.  Check it out here: http://blogs.msdn.com/bkrieger/archive/2010/04/27/visual-studio-client-connection-issues-after-upgrading-to-tfs-2010.aspx