Valve chose Ubuntu because it is "still experimenting" and Ubuntu is a popular distribution, but it will look to support others in the future. Right now the Valve Linux team is working on getting the Steam client on Linux with full functionality, optimizing Left 4 Dead 2 with a high frame rate and respectable graphics, and porting additional Valve titles. There will be an internal beta of Steam on Ubuntu in the "near future."
The Valve Linux team will post updates on its blog, complete with penguins.
Players generally have to use a compatibility layer such as Wine to play Steam games on Linux distributions, a process that can lead to some unwanted, unwarranted complications, no matter the platform.