This has been a long time coming, but with opening Greenlight where fans can influence which games are developed another new direction is being taken by Valve! The Steam and Source game engine will be moved to Linux in particular Ubuntu, the reasons for this move is that the variables in testing are less, its faster and is popular within the gaming community, after all gaming is what Valve is all about, so why not?
According to a posting on the Valve Software blog by the Valve Linux team, this is what is happening;
For some time, Gabe has been interested in the possibility of moving Steam and the Source game engine to Linux. At the time, the company was already using Linux by supporting Linux-based servers for Source-based games and also by maintaining several internal servers (running a 64-bit version of Ubuntu server) for various projects. In 2011, based on the success of those efforts and conversations in the hallway, we decided to take the next step and form a new team. At that time, the team only consisted of a few people whose main purpose was investigating the possibility of moving the Steam client and Left 4 Dead 2 over to Ubuntu.
Source [Valve Software]