A new video interview on PC Games.de talks with Valve honcho Gabe Newell about DOTA 2 and Counter-Strike Global Offensive during Gamescom. At around the 15:00 mark they ask if Counter-Strike Global Offensive will support cross-platform gaming like Portal 2 does between PC and PS3, and he explains that this is their goal:

That's certainly our goal. We never want to create these 'islands of players.' You have to be sensitive to control differences, and make sure you're accommodating that, but, you know, our view is players should be able to play as part of a community on whatever device they can. You know, sometimes there are contractual, or other issues that make that hard, where somebody, a platform holder doesn't want you to do that for whatever reason, but that's always out preference. is to let customers play and be part of the larger community.