Back in April, rumors about Valve developing a native Linux Steam client, as well as Valve porting over the Source Engine, were confirmed by someone who toured the Valve offices. Well, it's officially official: Valve is working extensively on Linux. They have recently launched the Valve Linux Blog, where they'll be discussing any developments relating to their software on Linux. The main focus as of now is a fully-functional Steam client, optimizing Left 4 Dead 2 to run with OpenGL (like DirectX except not DirectX), and porting over other Valve titles.
Currently they're working on all of this for Ubuntu, but if that is successful they'll most likely move on to other Linux distributions. As Ubuntu is one of the most popular distributions, they figure it's a good starting point. It's also easier to work with only one distribution than to have to work with many different ones.
You can read the first blog post for yourself here, and I'll try to keep you updated on any new posts.