Regarding processing power, I don't believe we can say the 360 can't run the game... There are games with great graphics running at 60 fps on the 360 (e.g. Ninja Gaiden 2), while MGS4 only requires 30 fps. Going by the information posted by Quaz51, and taking into account the framerate differences, NG2 is actually calculating 66% more pixels per second than MGS4, which means even if MGS4 graphics are much more demanding in other areas (texturing, polygons etc), there's room left for graphical improvement by having the framerate at 30 fps. Of course, there are many unknowns here, but my point is that it's certainly not proven that the 360 couldn't handle MGS4 graphically.
Regarding disc space, we can't ignore the uncompressed audio element. It's unnecessary for the vast majority of people, who have neither the ears nor the audio hardware necessary to notice the difference between well compressed audio and uncompressed audio. I'm pretty sure that if Konami was having difficulties with cramming MGS4 onto the Blu-Ray disc, audio would have been one of the first things to be compressed (even if with a lossless compression algorithm like FLAC which can halve the size of audio files with no loss of quality at all).
Being a software engineer, I can tell you one thing - it's rare for developers working in a professional context to optimize things which don't need to be optimized. I believe Konami wasn't space constrained with this game, having 50 GB to work with they took a lot of liberties like using uncompressed audio. If Konami is interested in porting it to the 360, they'll have a reason to optimize for lower space, at which point they can drastically reduce the size of the game data (most likely, starting with the audio as I said).
After reducing the size of the game data, in a game of this genre it's not hard to split the game between several discs (I'd venture the guess that no more than 3 double layer DVDs would be necessary). It's not an open game like GTA4 in which you can quickly go anywhere in the game. As many have said, it would actually be less painful for users to switch discs than to endure the multiple installations which the PS3 game requires.
Make no mistake - I'm not saying it would be a cakewalk to port this game. It's a big production, a huge project, and there are certainly a lot of things which would need to be done in order to convert it to any other platform. What I am saying is that, looking at the big picture, it's not implausible at all that this game can be ported to the 360. All the people I've seen attempting to prove otherwise are relying on unproven assumptions which only Konami itself can judge. Of all the publicly available information about this game, I have seen nothing which automatically proves the technical impossibility of a 360 port.
