Although its a task you seldom would want to do, it might become a necessity when someone leaves the team on hurry/short notice. The way out is aptly described by Matthew at http://blogs.msdn.com/mrod/archive/2007/01/08/undoing-a-checkout-that-belongs-to-another-user.aspx , and I just thought I would mirror the command to give it more live search-ability :)

tf undo /workspace:OtherUserWorkspaceName;OtherUserName  $/Project/..../FileName.cs /s:http://yourtfsserver:8080

 

Also, much more details available at James Manning's blog