Twin Reaper:
you might find it interesting to note that the TCP/UDP for pc and Xbox360 are almost exactly the same, and are used frequently by the media share system on the Xbox to read music and movies from a network connected pc. so the two will talk to each other, and any program would not need to translate any of the data as it will all be sent in the same format so there is not extra lag (it was originally Microsofts plan to let pcs on to Xbox live). also the core components (protocols, code interpreters, Direct X 9.0c) of an Xbox360 was based off windows XP. So I do not see how it is not feasible at the moment. Note: Shadowrun 2 is cross platform, Portal 2 is cross platform with PS3 as well.
Sentri:
The problem with Halo one was the fact that Xbox used DirectX 8.0 for its Drawing API where as PC used at that stage 9.0 and 9.0 was not very backwards compatable (induced large amounts of lag for the games complexity) so it had to be re written by Gear software for DX 9.0. The Xbox360 now uses Direct X 9.0c and pc 9.0, 10.0 and 11.0 which are fully backwards compatable with 9.0c.