Valve is one of the biggest publishers to throw its support around non-Windows operating systems. Today, they've released the Steam client for Mac OSX. In the coming months, Linux will be fully supported. I believe we'll hit some type of critical mass for Linux support and Valve is doing a great thing in supporting the free (as in freedom) OS.
Steam is a double-edged sword. It is a DRM system (albeit, one of the better ones) which means that I really don't like it that much. DRM takes freedom away from the consumer. We'll see what happens in the future with Steam on Linux.
Hey Blizzard, are you listening? Starcraft 2 for Linux NOW!