Writing on the official blog
the developers have noted that L4D2 is still coming to Linux! Which is great news since it was supposed to be the original title!
We are hard at work on the Linux release. While we don’t have a solid ETA yet, we are internally testing on Linux, MAC, and PC. Right now, it is looking pretty good. We will have more news as we get closer to release.
So we should see it by the end of the year along with hopefully Counter Strike: GO and DOTA2!