[Phylum]

They are compatible, though.

There's no hard reason cross-platform multiplayer couldn't be done. There might be some hoops to jump through, but in the end all the client has to do is interpret the packets it gets sent from a server. There might be more complications if Xbox Live has some form of control over how servers are hosted, but as long as there's no Top Secret Xbox Verification I can imagine it being completely prohibitive.

I'd be interested to know how this is controlled at a Sony/MS level.

e: Cross-compatibility would probably mean that you couldn't use the PSN/XBL apis, so that could potentially put devs off too. I can't say much because I don't know how the systems work, but that could become hacky pretty quickly. It's a bigger problem when you have client hosted servers and I don't even know how much PSN and XBL support that.