"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 revealed to Joystiq. "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."
If you would like to get into the ongoing Counter-Strike: Global Offensive beta, fill out Valve's survey. If you want cross-play back in the game, wait to sign one of the dozens of online petitions currently being created.