Well, after about 6 weeks as an invite only Beta, Valve has opened up to the public. Steam is now available to Linux users who run Ubuntu 12.04 as stated on their site. I imagine if you are running 12.10 it should also work, but due to 12.04 being an LTS, that was their target.
Clicking around the site suggests they about 39 Linux-supported games in their catalog of 2000 games. The Humble Indie Bundle has done wonders in the past (and still does!) for bringing games to Linux users, and with Valve stepping up, gaming will continue growing for the OS. Being a Ubuntu user myself, this is exciting news for me!
Give a look over at OMG! Ubuntu! to see their series on the topic.