This article at Gameplayer asks the question "Could Rockstar's big gun decide the console war" in which they argue that a chain of events suggest that Microsoft are worried that GTA IV could see the Xbox 360 lose the fight.
"We heard something the other day, a line of dialogue that must scare Microsoft to death. It came from the mouth of a twenty something electrician as he installed a car stereo for a mate. He said "I bought my PS3 to play GTA IV".
That wasn't the comment of a fanboy, or an educated decision by a gaming enthusiast who had weighed up all the pros and cons of the two systems: it was simply all he knew. There was a GTA coming out, so he went and bought a PlayStation. Microsoft is desperate to change that mentality, but what started as a broad plan has seemingly deteriorated into panic. Here's how…"