Just lost a couple of hours to a curious issue with TFS.
Due to a recent brute-force hack attempt, we had to change all our AD user accounts to something more sophisticated are more resilient to dictionary attacks.
However, the change of user names caused some issues with TFS.
One of our developers found this magic bullet
tf workspaces /updateUserName:OldUserName /s:http://silo18tfs:8080/tfs/defaultcollection