Hideo Kojima says the only reason that Metal Gear Solid 4 is debuting on PlayStation 3 is that he likes to be first with everything, and that the last year's focus on MGS3 meant Kojima Productions didn't have any time to get to grips with Xbox 360. Actually, he says, it could quite happily work on Microsoft's system.
When asked whether MGS4 could work on 360 during a recent 1up interview, he said: "Yes, that's technically possible." It's not that he doesn't like Xbox 360, see, he just doesn't want to do things second. So he's a bit like those kids who "hate" bands because their mates found out about them first.
"Kojima Productions has this philosophy that we want to be the first in everything regarding new technology. So after we finished MGS3, the option was to work on the PS3 to try all the new things for Metal Gear Solid 4. The only reason for this is I'm the kind of person who wants to be the first person on the moon, and don't want to be asking 'what the moon was like?' from other people." (Shh, don't tell him.)
So yes. Metal Gear Solid 4 could work on Xbox 360 ("maybe some nuance or a small detail here and there might be different") and the reason for that, apparently is that "there are really no differences" between the PS3, 360 and PC, although obviously Revolution is "totally different".