Last month Valve announced that it is going to release a beta version of Steam for Linux. It would only have 1,000 openings for experienced Linux users to help debug in the first phase of testing. The beta should include the Steam service and one game. Nothing else will be available until possibly the second phase of testing.
So, If you have Ubuntu 12.04 or newer, are an experienced Linux user and you have Steam account, get on Valve’s site and sign up for the beta.