Hideo Kojimo told Famitu magazine that the next installment in the Metal Gear saga will be a Playstation 3 exclusive. Even though previously Kojimo expressed the PS3 maybe a bit much for some developers, his loyalty for the Sony platform made this an easy decision.
“Metal Gear Solid 4 is only scheduled for release on the PlayStation 3 at the current time. Of course we could port it to other consoles, but doing so would require additional staff and research and development costs. If I were to make games for the Xbox 360 or the Revolution, I'd rather create original titles that match each console's audience, instead of the Metal Gear Solid series.” -- Hideo Kojimo
It doesn't sound like he has discounted the possibility of other Metal Gear titles for Xbox 360 or Revolution.