Valve has made good on their promise of a Linux-based Steam client and has opened the doors for applicants to register for access to the beta build. If you’re an experienced Linux user, you can head over to the survey on Valve's page now. Make sure you have a Steam account first, though!
If you’re not a highly-experienced Linux user with Ubuntu 12.04 or above, Valve would recommend that you sit this earlier round of testing out as the release only includes the core Steam client code and one Valve game, Left 4 Dead 2. Later releases are expected to be more feature-rich, so don’t worry if you don’t get in on this round.