Still very much in beta.
By Tim Colwill on January 15, 2014 at 4:44 pm
Valve have announced that the Steam beta client now includes the “-vr” command line argument to launch the software in their new, experimental VR mode.
After launching Steam, hit the Big Picture button to enter VR mode.
If you’re having trouble, try following the steps listed here on the Valve forums. Enjoy!