We have a project that is stored within our TFS server and some files were Checked-Out by me from another computer and another user (both of which are not used anymore). Is
I was able to undo another user's checkout with the following command:
tf undo {file path} /workspace:{workspace};{username}
You'll need to wrap that semicolon in double-quotes if you're running the command from PowerShell. We're running TFS 2010 (and VS 2010).
Disclaimer: I got this from the FCI-H blog at http://fci-h.blogspot.com/2011/01/how-to-force-undo-checkout-tfs.html