Defiance, an MMOFPS which will have an influence on the SyFy series of the same name, could have been the first game officially released to pit Xbox 360 players against PS3 players. In a private demonstration, developer Trion Worlds displayed how easily it could be done, showing two played on opposite consoles fighting one another in real-time.
Unfortunately, Microsoft has blocked the idea from ever coming to fruition. Despite the fact that it would be a fairly simple thing to implement, brand director Alex Rodberg made it clear that it probably would never happen: "Microsoft won't let Sony players play against them."
And this isn't one of those things that gets said and disappears without comment. Microsoft has issued a statement confirming their position: "We have a high level of expectation for our game developers to ensure that all Live experiences remain top notch. Because we can't guarantee this level of quality, or control the player experience on other consoles or gaming networks, we currently do not open our network to games that allow this cross-over capability."
So if you were ever hoping for either PS3/360 matches or Steam/PC/360 matches, it's not going to happen for the time being. Other than that, Defiance looks like it'll be pretty good and it'll be interesting to see how the game comes to fruition over the next few months.