The Undoer said:
I heard Left 4 Dead series was being ported over to the Mac OS, so I say get them when it's out.
AS has already been said Valve is bringing steam to Intel based Macs, including ALL its source based games, including HL2+episodes, Portal, L4D, LFD2. All those with multiplayer on the PC, will have it on the Mac and will play on the PC servers against PC and Mac players.
Also its a one account for both platforms deal, and one game for both platforms, that means if you own any of them, or buy any of them in the future, you'll be avble to play them on both a PC or a Mac.
Blizzard is the only other major developer with a similar development ethos with regards to both platforms. Most others if their game appears on a mac its becasue someone else ported it on license from them, and you have to buy a copy for each platform, AND you tend to not have cross platform play.