After the Steam UI update some folks on the steam forums found OSX specific images and such hinting at a release of Steam for the Mac platform. News today all but confirms that hint:
http://www.macworld.com/article/1468...valve_mac.html
and
http://kotaku.com/5484948/valve-teas...-not-so-subtly

And based on the images (which isn't the best idea), here's to hoping they're actually implementing a wrapper or straight porting over the Source games.