I heard rumors about it before however that is merely speculation. Won't believe it until Kojima says it himself.
EGM started this rumor almost 2 months ago -- but apparently, their article didn't provide any sources, quotes or reasons for the move. Konami then made a statement denying the rumor. Of course, by saying "Konami has no plans to bring Metal Gear Solid 4 to the Xbox 360 at this time", they are saying that it might end up on the 360 eventually, just like GTA did.
I pray, i mean i pray they do, if this and halo are released near the same time, you will possibly never see me again
old article, but at least it is more than an uncredited rumor. http://www.gamepro.com/news.cfm?article_id=55468