Ugh. Sounds like mostly working around design decisions and infrastructure. Getting the engines talking to each other sounded straightforward.

Technology is just one challenge. Once you're over that there are fundamental platform differences you can't gloss over.

Specifically, for skill based multiplayer games (such as most FPSes) the advantage of a keyboard and mouse over a controller is hugely significant. To a degree to where it might as well be cheating. The inevitable result of cross platform play becoming available between PCs and consoles in games like Black Ops, Halo: Reach, etc. would be that gamers quickly segregated themselves by platform, to keep games interesting.