As reported Rockstar landed a major sales record with Grand Theft Auto IV with $500 million in sales the first week. From the 6 million GTA IV copies sold apparently 60% are for the Xbox 360 console.

Microsoft's GameScoreBlog published retailer statistics that also show a strong sales increase in Xbox 360 console sales. Xbox 360 sales increased by 54% in the week of the GTA IV launch compared to the previous week.
About 40% of the newly sold Xbox 360 consoles are in combination with a GTA IV game.

The Xbox 360 still has a larger install base compared to the Sony PS3, which could explain the larger share of GTA IV Xbox 360 copies.

Also interesting is that 2.3 million people played Grand Theft Auto IV on Xbox Live in the first week making it the number one game on Xbox Live, see you later Halo 3.
More details on the GameScoreBlog. Via GameSpot. More details about GTA IV.