From this article on geek.com:
A Phoronix reader by the name of Joe Davison decided to e-mail Gabe Newel and ask him if the Linux version of the Steam client would be made public before the end of 2012. In a very Steve Jobs’ like response, Gabe simply said “Yes.”
That means at some point in the next 7 months anyone running Linux will be able to download Steam and start playing a number of games, including at least one Valve title. After that the emphasis will be on game developers to start porting their Steam games over to Linux.So for those of you who, like me, use Linux as your primary operating system, we'll finally be able to use our preferred OS for playing some of the awesome Steam-powered games like Left 4 Dead, Portal and others!