Many online publications, GamerNode included, recently reported that Konami’s Metal Gear Solid 4 may not be as exclusive to the Playstation 3 as previously thought.
In response to these news stories, Konami has stated that everything is merely "speculation." They did not DENY the veracity of the stories, but dodged confirmation or non-confirmation with thier well-chosen words.
The wind up of it all is that there is still quite the possiblility that MGS4 will show up on the Xbox 360 at some point, remaining exclusive to the PS3 for only a certain length of time.