I recently ran into the problem where I needed to rename some folders in source control of my team system project but in order to do that there cannot be any files checked out. I knew the developer with some checked out files was not doing any changes but also wasn't around to undo the check out so I needed to undo them myself. There is no way to undo someone else's check out through the Source Control Explorer in the VS2005 IDE but you can do it using the tf command line tool.
I found a great blog entry explaining the commands. http://blogs.msdn.com/rimuri/archive/2006/03/06/544686.aspx
One thing to remember is if you aren't running these commands on the TFS server itself then you need to add the /server:servername parameter to each command to tell it what server to run against.