Here's an article written by a dude who was brought in to fix some bugs, previewing official Linux support for Steam and Source.
In case that site's down (it's currently suffering on my end), here's a quick summary: They're currently focusing on Left 4 Dead 2, because it's a stable codebase. More Valve games will come soon. They're doing entirely native ports, no Wine or other emulation wrappers. The release date is "not too far out", but it'll likely be as a public beta limited to just L4D2. While his exact words weren't published, Gabe's apparently surprisingly enthusiastic about the OS and extremely negative about Windows 8... Interesting times, huh?
There's also some photos of L4D2 running natively in Ubuntu. Here's one of them!