If the game hasn't already been in development for the 360 I doubt we will ever see it. I think it take just way to long for them to port it over. I think once u get past the 1 year mark for a port it simply wont do well.
According to Jonnyram at GAF, the MGS4 port on 360 is "the worst kept secret in Japan" or something to that affect.
In other words I'd expect it's already in a pretty advanced state before they start talking about it. If it's true, they've likely been beavering away on the 360 version for a good while already.
We're simply in a state where next gen games are so expensive, it's nearly impossible to justify excluding either PS3 or 360's userbase. Big third party exclusives are nearly gone, and you can thank 360's one year head start for a lot of that.