You wouldn't know it by going to the Steam home page, but you can in fact download Steam for Mac now—the secret link is live. Click here. Update: AND, here's how to start playing immediately:
1. After you've installed Steam for Mac and gotten it ready to go, go to Steam's Mac beta page.
2. Click on this Portal ad in Chrome or Firefox.
3. Tell it to open the link using Steam—your Portal download will begin immediately.
4. Rejoice. To get to the other games, check the box for "view games library" or click "update news," which'll bring you to the main Steam screen.
(And download TF2 once Steam servers stop being busy, so I can shoot you in the face.)
5. BTW, this only works once—if you have to close Steam, you have to reopen it with this trick.
6. We opened Portal and started playing just fine, though we're hearing you might get booted again once you try to open the actual game. Worst case scenario, you'll have the files ahead of everybody else. (A good thing come the rush tomorrow.)
6a. Be disgusted that Steam stores all of your game data in the Documents folder. WTF, Valve?
So. Excited. I just hope Steam's servers stop melting. [Steam via Geekosystem, Thanks ninjapig88 for the Steam trick!]