It's rumored that Metal Gear Solid will no longer be a PlayStation exclusive and that the fourth edition of the game MGS will make its debut on both the Xbox 360 and PC.

Sources at Konami says that they are working on putting Metal Gear Solid 4 on the Xbox 360 and PC. However, Konami has been working with Microsoft so the 360 and PC versions will both take advantage of Xbox Live and Games for Windows Live, with such features as achievements and the ability for 360 and PC owners to play against each other in multiplayer. However, they do not have an exact release date for this game.

That is all that I have read, but if you want to know more, tune in on July 11th for the next E3 show.
Then again, keep in mind that this is only rumors.