The only reason why I do not play FPS on any console is due to the controls. If they allowed keyboard/mouse controls on consoles then there would be no issues. I do not know why Sony/Microsoft will not allow keyboard/mouse controls in their games, when each console does support mouse/keyboard input already.
I played Final Fantasy 11 online, and that worked fine on the 360. That was cross platform too, you could play your characters on PS2, PC or 360 - and all platforms played on the same servers. The only thing new about this is that the game runs on a mobile platform, but cross platform MMORGs are not new...