Remember the external beta Valve planned to roll out sometimes in October for 1000 users? This is that. It includes support for Ubuntu 12.04 and above, and a beta version of one existing Valve game.
Which game that’ll be remains to be seen, but it’s likely to be either Left 4 Dead 2 - already promised Steam Workshop support on Linux - or Team Fortress 2, if Steam’s Content Description Record is anything to go by.
Valve are calling for experienced Linux users, so if like me you’re merely an enthusiastic novice I’d advise waiting it out for the next beta.
Otherwise, apply away. The developers have previously mentioned that Left 4 Dead 2’s zombies actually run faster on Linux, so I imagine that’ll be a sight to see.