The developers of upcoming MMO The Agency, Sony Online Entertainment, are releasing the game on two platforms. PC and PS3. So it's natural to hope that they're looking at cross-platform play between users of either system, yes? Correct. Natural and fortunate, because that's exactly what they're doing. Replying to a question asking about play between PS3 and PC users, SOE's Matt Wilson replied "we're actually doing that at the office. Technology-wise, it's really not that complicated". Good news! Temper that good news, however, with a pinch of pragmatism; just because they can do it, doesn't mean they will. There's all sorts of things to consider, from the ease of hacking/cheating on PC to differences in control schemes to the inevitable "that's not fair" whining that comes when one group of players is using a control system they think isn't as good as another.
SOE's Wilson Talks Connected PC, PS3 Game Worlds [Gamasutra]