The creator of the Metal Gear Solid series, Hideo Kojima, has officially released his statement to all your questions about MGS4 being released on XBOX 360.
Metal Gear Solid's success is believed to be linked to the success of the PlayStation console, and therefore, the new release of the game - Guns of the Patriots - has been completely optimised for PS3, and would "probably not be possible" to port the game over to the 360.
Hideo Kojima has definately shown interest in producing the game on other consoles, partly because of the sheer profit he would generate, and partly because of the (lack of) success of the new PlayStation 3 compared to the rival XBOX 360 and Nintendo Wii.
One of the pet hates that Kojima has discussed is the fact that people are looking at consoles' hardware issues rather than experiencing the gameplay, and has stated that he "feels like it should be made for PC, so that people don't ask him about hardware platforms."
To get to the point of MGS4 being released on 360 - Kojima has "no problem with it", but feels that it wouldn't work in this case. He says in the interview that he would like to follow in the footsteps of companies like Capcom, who make games for the PC platform, which can then be ported to Playstation 3 and Xbox 360.
The whole interview with Hideo Kojima can be found here: http://www.eurogamer.net/article.php?article_id=225581
