Ryan Payton has said in a Kojima Production podcast that MGS 4 is being built exclusively on PS3 hardware to the specifications Hideo Kojima planned from the start.
"I can tell you honestly at this point, we have not changed the game in any form with the idea of 'oh, we got to do this on 360.' Put aside whether or not we're developing for 360 or not, the game is built on the PS3 hardware, as you guys have known for the past two years or so," he said as reported by 1up.