Batman: Arkham City website was recently updated with details on retailer specific pre-order bonuses. It appears GameStop has the best hook-up at the moment, offering "Joker's Carnival Challenge Map" in exchange for your down payment. The only other bonus noted at this time is Best Buy's nondescript "combo pack."
We contacted WBIE for details this morning, but have yet to receive any specifics. As always, we'll update when we learn more.