Wow... just reading the OP here, but I just feel like clarifying the logic, cause people just love to play around with the numbers without actually thinking about what those numbers mean.
A highly rated game just came out. A game that makes use of enough of the power of the HD consoles that makes packing all the extra tech in there really seem worthwhile. This game has been received in much the way Melee had been for the Gamecube; It's the game that makes it worth the purchase of the console, even if you only play a couple other games irregularly.
For anyone planning on getting an HD console, but holding out, this is going to be seen as the point at which it's finally worth making the purchase.
Now lets look at the current standings; 360 has close to 12 million console in america, and PS3 roughly 5 million. With over twice as many people owning a 360 already, that means twice the number of people who don't need to get a console just to be able to play this game. It's only natural that the group with more people buying consoles would be the group that has less console already sold.
However, there is the obvious point to consider in that the 360 version has been rated consistently lower than the PS3 version, which may also be adding to the issue.
Another point is that while there's episodic content for GTA IV on the 360, there's no certainty that it couldn't make it's way to the playstation network, even if turns out to be a delayed exclusive for teh 360, and on top of that, I don't think any of that episodic content is out yet, so until that point, you're unlikely to see a surge of people getting the 260 and the game for that purpose.