DLC, possibly as early as this summer;
http://kotaku.com/385271/gta-iv-dlc-arriving-this-summer
Rockstar's "Lazlow" (one of the game's DJs) made another appearance on the Opie and Anthony show this morning. This time he claimed that the Xbox 360's DLC is being worked on at the moment and could see the light of day as early as this summer. Not much was said about PS3 DLC other than he hoped to see it happen in the future. Our guess is that Microsoft will release the DLC whenever it works best with their general plan of attackbe that as soon as Rockstar finishes the content or months latersince paying $50 million and ruling a platform probably affords you such luxuries.
http://kotaku.com/385271/gta-iv-dlc-arriving-this-summer