Technically it is possible, the games for the different platforms are written with the same game engine, therefore the server software for the different platforms is the same.
And there is no reason the platforms could not communicate with each other.
They just need to hook up the different server networks with each other, and that's it.
No fancy code rewriting necessary.
Just like they did with the servers for each platform (There isn't just one server per platform, rather a whole server network per platform).
The only restriction is that in some lobby's there can't be more than 16 players online, because xbox 360 and ps3 consoles don't support it.
So yes, it is possible and not that difficult to implement, but Sony and Microsoft don't want that to happen, due to the console war that's going on. Therefore the gamers are screwed basically.