It appears that Metal Gear Solid 4 may very well have joined the ranks of all the other titles that were once PS3-exclusive.
According to a mildly clever post over at destructoid.com, the game MIGHT possibly be heading to the Xbox 360 in the golden month of November 2008. This is just about one full year after the scheduled release date of MGS4 on the PS3.
Considering the track record of MGS exclusivity, a delayed release on the Xbox 360 seems a perfect fit. All previous MGS titles remained exclusive for a limited time, as well.
Of course, the information above is all just SPECULATION, but phone calls and legal contracts that spell things out in just the same way are usually not.