Well, Valve sent out some info on getting Steam on Linux. According to the blog, Valve has Left 4 Dead 2 working natively on Ubuntu Precise (12.04). Also, they have managed to port Steam to Ubuntu, with most features.

Quote:After successfully porting L4D2 to Ubuntu, interest grew within Valve and, as a result, the team and projects wee were working on also grew. Currently, our focus is on the following projects:

• getting the Steam client onto Linux with full functionality
  
• optimizing a version of L4D2 running at a high frame rate with OpenGL
  
• porting additional Valve titles