Nice find it's sad they have the capability to do this but are not allowed to run cross platform at launch.
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Portal 2 also had cross-platform play between PS3 and PC, although that's a slightly different setup (only two at a time). It worked perfectly fine. On xbox players were stuck by themselves. It's definitely Microsoft that tends to be the sticking point for this kind of thing.
There was cross platform for FFXI which is a mmo was alot of fun. Only problem that tend to happen was xbox would freeze out in high traffic fights. I think that is the real reason because it can't keep up with all the gameplay. I played FFXI on pc and 360 was fun either way. Playing on the 360 was really comfortable to where the pc ran it better. But also FFXI code was horrible and they kept just dumping on top of it instead of keeping it clean.