When Counter-Strike: Global Offensive was first announced, Valve had said it would include cross-platform play between the PlayStation 3 and PC versions. However, due to the PC and Mac versions’ frequent post-launch updates, that is no longer the case.
“The beta has proved we want to update not just the beta, but the game itself post-launch frequently on the PC,” Valve’s Chet Faliszek told Joystiq (via RPS). “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.”
A PC beta for Global Offensive is ongoing. If you want in, try filling out this Valve survey and hope for the best.