Source SDK 2013 update gives modders "support for virtual reality via the Oculus Rift"
A Steam news note announces the arrival of an updated version of Valve's software development kit, which grants "support for Mac OS X and Linux to mod developers" and adds "the ability for virtual reality support in your mod." Yes, expect to see a wealth of Oculus Rift mods heading to a Source game near you. Ricochet with Oculus Rift support! The dream lives.
There have been other alterations, too. The source code is now up on github and a tweak to the license agreement allows users to share modified versions of the kit for free. If you're interested in making mods, the Valve Developer Community wiki is a good place to learn.