Semi-educated guess: no, won't happen. While the details are semi-secret and probably covered by NDA after NDA, it seems like XBox games are more or less forced to use the LIVE network (Exceptions being MMOs). It's part of that 'Gold membership required for Multiplayer' marketing stuff. And LIVE is deeply incompatible with the way the game works on the Mac. It's possible to use Games For Windows LIVE, which is compatible with XBox LIVE, but still, that leaves the Mac out, and it seems to be available only to very special MS partners anyway, judging by the almost vanishing number of games that have come out over the past three years using it.
So either way, a special touch with MS is required to make this happen. IV's relations with MS are generally good, so all is not hopeless; but personally, I'm not holding my breath.