Valve is expected to have the initial Steam Linux client available along with the Left 4 Dead 2 game in some form of beta soon, full-blown Linux push by Valve Software might not happen until around February of 2013.
Valve has already been showing off Steam on Linux to partners and Left 4 Dead 2 is in decent standing: L4D2 is faster on Linux than Windows with the NVIDIA binary blob and on the open-source Mesa driver its already running on Intel hardware. We also know that Serious Sam 3 is coming to Linux thanks in part due to Valve, plus other announcements are likely forthcoming from Valve and other game publishers.
With all of Valve’s modern games running off the Source Engine, which is now natively working on Linux, going from L4D2 to other titles like say Counter-Strike: Global Offensive, Team Fortress 2, and Half-Life 2 won’t take as long as starting from scratch.
With that said, seeing a larger catalog of their titles natively available on Linux around February does seem rather reasonable although this information didn’t come to me from their Bellevue headquarters directly.
For some reference when Steam came to Mac OS X, Valve announced that in early March of 2010 with plans for release in April of 2010 but in reality the post-beta launch happened in mid-May. At launch there were around 50 games available through the OS X Steam client while that count has continued to increase over time.