The Left 4 Dead 2 Cold Stream now says, "Later this year, L4D2 will be available on a brand new platform. It will be one of the first Valve games to be released on Linux. This will allow Linux users to run Steam and play the game directly without having to run additional software. You can follow the development HERE and we will have more information later this year as we get closer to release."
There's also a photo of zombie-ified Tux: