Edited to add:
NOTE: These instructions are out of date. Please find the latest instructions here
As of the most recent Steam Client Beta steam now supports an experimental VR mode. If you
own an Oculus Rift dev kit you can try it out by starting Steam with "-vr" on the command line. Then press the Big Picture button to enter Big Picture + VR mode.
The first time you run you may need to do the following:
1) Run Steam in the desktop client without the -vr option
2) Find "SteamVR" under "Tools" in your library. (If you don't have it installed, install it.)
3) Bring up properties on SteamVR and opt-in to the "Beta Update" beta. Let the update download.
4) Quit the Steam Client again and start it with -vr
Please use this forum to provide feedback on VR mode.
Edited to add: This feature is currently only supported on Windows. Linux and OSX support are on the way.