On Topic here are why I think it will be PS3 exclusive:
-Sales are above what they expected
-Sony advirtsied ingame
-File Size
-They have stated that it is a PS3 exclusive
-MGS sold poorly on the Xbox when it was ported
-Game supposedly took advantage of the Cell that hasn't been sued before; making it harder to rewrite code I imagine(I have no expierance on this so someone correct me if I'm wrong).
-People who really want MGS4 prob already shelled out cash for a PS3, further hampering 360 sales of the port
-Microsoft would have to pay for the port(which they won't).