The rumors can finally be put to rest, as Nightwing will indeed be a playable character in Arkham City though not right away. He’ll be sold as part of the games first DLC, appropriately titled the Nightwing Bundle. The challenge pack will include the maps Wayne Manor and Main Hall, with Nightwing as a playable character for the Challenge modes.
Robin will actually be in the DLC as well, making him available outside of pre-ordering the game at Best Buy. The DLC will release a few weeks after the game for 560 Microsoft Bat-Points or $6.99.