The official Batman: Arkham City Twitter account has a Batchannel update for all of those who want more environments to debilitate their enemies in. The official tweet
Brand new DLC coming December 20th with all new Batcave. Jokers [sic] carnival and the iceberg lounge also available then!
Pricing details were lacking from the tweet, but hey, we're going to visit the Batcave! We'll update you when we know more about the DLC pack, but until then keep your fists primed for action.