While there exists bypasses so individuals can launch the Steam Linux client without their account being one of the ~1,000 beta testers, you need to be part of the selected group to access the Source Engine games that have been ported over to Linux. The beta began last week but Valve Software's Linux developers say they have been regularly adding more people to participate. "We are adding more participants on a regular basis."
You can apply to be part of the beta program based upon the Valve Linux survey with their interest primarily to have a diverse select of testers / hardware configurations.
I am part of the Valve Linux beta but am not allowed to share any benchmark results at this time or any other coverage until the beta becomes more open with all of their initial bugs worked out. Stay tuned for more with Steam on Linux.
Valve's latest Linux blog post can be read here.