According to The Bitbag, downloadable content will be massive and Xbox 360 exclusive. They indicated the following:

* When MS says exclusive, they mean that no other system can have DLC content for GTAIV.
* It will be bigger than a lot of people think. GTA3 Vice City and San Andreas were basically expansions of GTA3. Think of the DLC that way. We are talking major expansion for GTAIV.
* No price has been discussed yet. It’s up to Rockstar on whether or not they will charge for it.
* If successful, DLC may be the new way to deliver GTA expansions.