Some information is out on the web about a bug that exists when creating a project that results in an error message similar to "The identity [Project Name]\Project Administrators is not a recognized identity" (to resolve that issue see James Manning's blog entry: http://blogs.msdn.com/jmanning/archive/2005/10/21/483682.aspx). However, I ran into this issue when my account name changed and not my physical account.
 
 
When attempting to commit my changes after my account name change I received the following error:
"The identity DOMAIN\MyUserName is not a recognized identity"
 
To resolve the issue, the following steps had to be performed on the workstation that was giving the error:
  1. Under the Release Candidate one could simply run tf /updateusername:DOMAIN\YourNewUserName and everything would work. However, I was running Beta 3 Refresh, so this syntax was not available.
  2. For beta 3 refresh the following commands must be run:
    1. tf workspaces /updateUserName /s:TFSSERVER
    2. tf workspaces /remove:* /s:TFSSERVER
    3. tf workspaces /s:TFSSERVER
 
If you're worried (like I was) that your pending changes would vanish, never fear! The /remove parameter works against a local cache file. The issue is that the /updateUserName parameter actually updates the workspaces on the server, but the cache file still has information about the old workspaces. the /remote:* command removes the workspace entries from the cache, so upon retrieving them again all is well.
 
Hope this proves useful.