As shared on the NVIDIA site, and I've since been able to confirm with Valve, the Linux beta of the long awaited Steam client starts today. This initial beta is limited to select participants while the bugs and early issues are worked out. As said already, the initial Linux target is Ubuntu 12.04/12.10. After the early issues are worked out and more games available for Linux atop the Source Engine, the beta will expand to more of the public.
Valve's also holding off on media/press coverage of this initial beta, so right now I won't be able to share the Linux GPU/driver benchmarks and requirements until a later date. It should work with the open-source Intel Linux driver while AMD and NVIDIA Linux users will be best off with the binary blobs.
More information later; pardon the terse posting from my mobile as in the middle of a Phoronix Test Suite planning summit.