Cross-platform multiplayer gaming is an interesting idea, and with the advent of modern consoles supporting Internet connections it's clearly possible. There are a lot of obstacles, however. For one, console and PC development are rarely done side by side, and keeping the games patched simultaneously could be a development nightmare. On top of that, more simplistic issues like the controls for a console not matching the PC version could inspire some to say that certain players have advantages over the other. Not to mention that people cheating in PC games online is a big issue, whereas it's rarely heard of in multiplayer consoles.
I don't see it as happening – but I hope it will, since it would tie two large pools of gamers together.