Actually, Konami will be shipping MGS4 for both Wii and 360 owners. The game even comes with a free PS3! It's true. You heard it here first!
=)
Seriously though, it'll be a long while before MGS4 ever makes it to the 360. A loooong while. The MGS4 engine is very PS3 centric -- that'll take a bunch of money to change. Konami may just opt to not do it at all. The number of buyers will be diminished by those who own both systems, and that's been shown to be a higher number than most believe. The profit margins may be really marginal for a rerelease game whose primary audience is mostly satisfied.
The same may be true for Bioshock on the PS3. Looking at the numbers for PS3 Lost Planet, and PS3 Oblivion, its clear that not anywhere near the same % of buyers are interested in re-releases. I would actually say that MGS4 is more fan-centric than either of those games (meaning less general appeal, and the title will be front-loaded with early buyers as opposed to having long legs), and this would further reduce a 360 port's profits, since many of the fans went out and scored a PS3 (many of them probably well in advance), rented the game, played it on a friend's PS3, etc.
I'm not saying it can't happen. I am saying that there's a lot of evidence that it won't, and that claiming that its "inevitable" is pretty out there, unless its from a *really* trusted source.