It is not as easy as it should be to undo someone else's checkout in Team Foundation Server 2005. In 2008, you can do it through a GUI, but only if you have the Power Tools installed.
tf undo [/workspace:workspacename[;workspaceowner]]
[/server:servername] [/recursive] itemspec [/noprompt]
I was also faced with having to add my login info, so the exact command I used was as follows:
tf undo /workspace:WORKSPACE-NAME;User $/myProject/CheckedOutFile.cs /login:myusername,mypassword
I also found out that you can delete an entire workspace for a developer who is no longer on your project and maybe had some files checked out. That command is even easier, and will automatically undo any checkouts:
tf workspace /delete Workspace (add "/login:name,password" to end of command if not using AD)