In a new blog post, Valve has announced that they will be starting internal beta testing for Linux client next week.
There will be an external beta as well in October for about 1,000 users. They will be providing a sign-up page for users to apply for beta testing. Any further information about this will be announced in a future post (we will keep you updated).
Beta testers will get access to one Linux game and Steam client. Only Ubuntu 12.04 and above will be supported.