Valve has confirmed that the cross-platform multiplayer between Mac and PC is no more.
Chet Faliszek told Joystiq:
The beta has proved we want to update not just the beta, but the game itself post-launch frequently on the PC… To do that we need to separate the platforms so one doesn’t hamstring the other. So for that, we have removed the idea of cross-platform play — essentially make all platforms stronger by not mixing them.
