A Steam Community post by Valve reveals that beta virtual reality support is present in the latest beta Steam client (thanks Road to VR via Gamasutra). If you happen to have an Oculus Rift dev kit, here's how to access SteamVR:

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