Valve has announced more details about the release of the Steam client on Mac. It's coming in April, will feature native versions of Valve's own games (such as Half-Life 2, Left 4 Dead 2 and Portal) and will share its multiplayer servers with the PC; don't worry about not being able to frag your PC-owning friends in Team Fortress 2, in other words. Also, future Valve games will be released on both the Mac and the PC at the same time, Portal 2 being first up later this year.
If you buy a game on the Mac client, or vice versa, you'll be able to play it on the PC client too - luckily you don't have to buy multiple copies.