Valve has already confirmed that Steam and Left4Dead 2 are coming to Linux. They have launched a dedicated blog for Linux where they will be writing about progress on Steam for Linux.
Now Valve has released a new Cold Stream DLC pack for L4D2. With this release, Penguins have also made their first appearance on L4D2 website.
Those who were wondering about expected release time, this only confirms that it will come later this year. So no release date yet.