The Team Fortress 2 blog has answered a bunch of questions about the just announced Steam-on-Mac which I've written about a couple of times. IE:
Q: Is it just some crappy emulated version of TF2? A: No! Also: How dare you! Mac users aren't getting a crappy emulated version of the game. TF2 will run natively on OSX, like an actual big boy game for adults.
Sadly it's not the full steam catalog, but the Source engine games. Still, that's a fair number of games and people like some people I know can worry less about their aging win-boxen's hardware.