Gabe Newell has publicly said yes to having Steam on Linux this year and that the Linux beta will be soon, but I am hearing now that their official Linux push might not be until 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 -- when factoring in Valve Time too -- although this information didn't come to me from their Bellevue headquarters directly.
I exclusively shared this information yesterday via Twitter.
Hopefully by February we will also see Valve acknowledge/support other Linux distributions besides Ubuntu when they and their partners begin pushing out more Linux titles over Steam.