While Valve has already released Half-Life 2 and most of their other titles for Linux gamers, Left 4 Dead 2 was one of the holdbacks, reportedly due to some performance issues. I had been playing L4D2 on Ubuntu Linux more than one year ago at Valve's headquarters as it was their first Linux port they were working on, but only this week have they done the non-beta public release of L4D2 on Linux.
It was in early May that Valve did a beta build of Left 4 Dead 2 on Linux. The beta tag is now removed. If you fire up the Steam Linux client you will now be able to enjoy this great first person shooter.