Report: Batman: Arkham City Story DLC Coming
Listings for leaked PS3 trophies may indicate future story-based DLC for Batman: Arkham City.
Website ps3trophies.org lists 10 new trophies for a Harley Quinn DLC that appears to be story-based as opposed to more challenge rooms.
Among the trophies is one for using "5 different Quickfire gadgets in one fight as Robin in Harley Quinn's Revenge." This would naturally point to Robin being a playable character in the DLC.