Rumors were rife earlier this month about a possible Harley Quinn DLC on its way for Batman: Arkham City which was leaked out with the release of a trophy sheet, a new image posted on the official facebook page of Arkham City seems to have confirmed the rumor.
We don't know whether she'll be a playable character, however, the trophy list did suggest Robin to be a playable character and the upcoming DLC to be a full fledged single player add-on, unlike the previous DLCs, which were just arenas and additions to the game's challenge mode.
Don't discount the possibility of her being a playable character, just yet.