Rocksteady has announced, via the Batman: Arkham Asylum infocast system
(an in game news ticker on the main menu), that "new free DLC will be available in 9 days". This is good news if you are among those that have killed this game within the first week of it's release and have not loaded it up since, or if you're simply looking for incentive to go out and purchase this 5/5 star Super Hero action title.
There is no set date on the DLC's release at this time, but 9 days from today will put it sometime next week. As to what the DLC will be, there is no news, but given the various exclusive DLCs, the Bat Armour that was available to Canadian customers of Walmart, the Challenge Map featuring the Scarecrow's army of skeletons for Gamestop pre-orders and the exclusive playable Joker for PS3, I'm sure this one will be tasty.