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. (And download TF2 once Steam servers stop being busy, so I can shoot you in the face.)
So. Excited. I just hope Steam's servers stop melting.