In one interview (in 2005 IIRC) he said that MGS4 could be done on 360 and slight hardware differences between PC, 360 and PS3 are no longer important. it seems he later changed his attitude toward platforms.hmm ... I thought he said it couldn't and pointed to the camo suite effect as an example.
As of MGS4 exclusivity, I'm sure it is secured by a deal between Sony and Konami and this is why Splinter Cell 5 is exclusive to 360 - it's sort of Microsoft's countermove. At least that's how I see it.