Well, the reason I say that it might be because of the 360 is because they have only shown the 360 version so far, and that version is REALLY buggy apparently, so it can't just be the PS3 that they're having problems with. I don't doubt that the PS3 is hard to develop for, but you can't tell me that MS would let a PS3 version that was less buggy see the light of day if the 360 version was having problems trying to compress everything, and it says in the article that Rockstar itself said that they were hoping MS would give them clearance to require an HDD to play the game on 360. That would imply that they are having at least a bit of problem with storage, and they can't be having that on the PS3, seeing as it has both blu-ray and a built in HDD in every unit.
