The game in question is Trion Worlds' upcoming Defiance, an MMOFPS that will interact with the Syfy television series of the same name. Trion showed a crowd how it's entirely possible for one player on an Xbox 360 to fight against or with another player on a PlayStation 3.
Trion Global Brand Director Alex Rodberg said that while this cross-platform mingling can be done, it's extremely unlikely that such a thing would ever reach live servers: "Microsoft won't let Sony players play against them."
A Microsoft spokesperson agreed with that sentiment: "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."