According to TVShowsOnDVD, marketing materials sent to retailers claimed that, while supplies last, "every DVD will include a 3-song download pack for the MTV Games sensation Rock Band."
It's not the first time that Rock Band and South Park have mingled, as the game contains a track from South Park band Timmy & The Lords of the Underworld. On the flip side, the eleventh season of South Park includes an episode that lovingly mocks instrument-based games, such as Guitar Hero and Rock Band.
No specific songs or platforms were named, though only the PlayStation 3 and Xbox 360 versions of Rock Band support downloadable content. A similar Best Buy promotion only offers an Xbox 360 download, though the exclusive Disturbed tracks will eventually be available on both platforms.