Left 4 Dead 2 is coming to Linux. I played L4D2 natively on Linux back when at Valve's headquarters nearly one year ago and have mentioned the L4D2 Linux port many times since. Valve employees confirmed last week the Linux port was getting closer to release.
For many months now, Valve has found value in open-source drivers and have been collaborating with Intel and their open-source team to improve Mesa. (On the AMD and NVIDIA side, it's mostly a binary driver enagement.) Going back to last summer, Intel has been committing performance optimizations for L4D2 and other Source Engine games with their Mesa DRI driver.
Most recently, Eric Anholt made a commit to Mesa yesterday for improving L4D2 with their driver. The commit is i965/fs: Improve performance of copy propagation dataflow using bitsets.
With this commit affecting a couple dozen lines of doce, it reduces the compile time of Left 4 Dead 2's slowest shader by about 18%.