Linux users can now sign up to help Valve Software test out the Steam for Linux client when the beta launches at some point in the (presumably near) future (via Joystiq). The company announced a planned Linux release for its virtual storefront in July 2012, with Valve's own Left 4 Dead 2 pegged as the first official Steam title to see added Linux support.
The short notice on Valve's website ask for experienced Linux users only to step up for the beta. In other words, if you just replaced Windows with Linux yesterday on a lark, you're probably not the ideal beta applicant. To sign up, head over to ValveSoftware.com and follow the instructions there.