It's likely to happen, especially considering the user base of X360 will still be much larger than the PS3 when it's finally released next year. Im tipping it will be ported in 2009.

As for the comment that MGS4 will require multiple dvds to fit on X360, that is total hogwash..MGS4 will have barely any pre-rendered cutscenes, it's all in game engine. Polygon size compared to video size is much smaller and could easily fit on 1 dual layer dvd. Unless Konami are doing fully uncompressed Dolby Digital Plus audio then theres not even a need for a Bluray sized disc.