In addition to being a system-seller, Microsoft asserts that the Xbox 360 version of GTA IV accounted for 60 per cent of total sales. It's likely the other 40 per cent purchased a PS3 as well, in addition to the multiplatform game, though Sony has yet to say by what percent.
Both Microsoft and Sony depend on GTA IV to sell consoles, as can be seen by the many platform-specific advertisements in circulation. Sony also releases their own to promote the PS3, with never a mention of the Xbox 360 version obviously.
As a testament to GTA IV's addicting missions, side quests, and free-roaming adventures, Microsoft closed Thursday's statistical announcement by saying that the average gamer spent more than four hours playing GTA IV last week, the most of any other game.