Now, I've heard through the grapevine on a couple of sites that MGS4 would be coming to the 360. Yeah, it would have been awesome, but I didn't give it much hope as Hideo Kojima and Sony have a lot of history together and I didn't think Sony was dumb enough to let one of their biggest franchises slip onto another next-gen console, because in the end, it's the games that people buy consoles for. However, this is the most substantial rumor I've heard regarding MGS4 on the 360, and just about every site has said that it's a go!
While it would be a stupid move on Sony's part, I wouldn't put it past them as, like I said, MGS2 Substance came out for the XBOX. It may have taken a while, but it eventually did. Now I've heard from more than just a few sites that MGS4 will be released for the 360 a few months after the PS3 release. The reason that Konami's been giving, supposedly: There's not enough PS3 units to make the game profitable solely on that console. If this is true, then you can chalk up another loss for Sony due to the Blue Laser Diodes and manufacturing errors. Check it out:
http://www.joystiq.com/2006/12/17/heightened-rumor-metal-gear-solid-4-not-a-ps3-exclusive/
UPDATE!: Well now, from what I've just read Konami has released an official statement saying that they "Have no plans to bring MGS4 to the XBOX 360 at this time.." Key words there, however, are 'at this time' meaning it will more likely than not hit the 360 eventually. I do recall them saying the same thing about MGS2 and the original XBOX way back when. Konami likes to do what is profitable at the time, and if the PS3 shortage keeps up, it will definitley be profitable to release it on the 360.