This question came up in our VSTS for Developers Training Course in Toronto this week. One of my students wanted to know how to undo or rollback a checkin, something you could do in Visual Source Safe.
Unfortunately this feature was cut for V1 of Team Foundation Server. Buck Hodges and Brian Harry from Microsoft share how this was a painful cut to help get V1 out the door in this forum post.
Update: James Manning (via the comments) points out that the Team Foundation PowerToy will do this for you. Great tool btw - check it out. Includes the following tools: Unshelve & Merge local changes, Rollback, Online (syncs offline changes to source control), Get Changset, and Undo Unchanged.