DLC seems to be the norm nowadays and without a doubt, it seems like it helps with the sales of ceratin versions of games. Much like Rockstar's last game, Red Dead Redemption, it will be included with exclusive content on the Playstation 3 version that the Xbox 360 version won't have, titled "Consul's Car Traffic Case."
There has been no official announcement of this at all, but a Rockstar Games representative has stated, "Look for more details on that, coming soon"