Eidos and Rocksteady Studios are priming Batman: Arkham Asylum for some free DLC next week.
"Free downloadable content coming in 9 days," confirms the news ticker below the in-game main menu of the Xbox 360 version. I took a picture as evidence.
UK PR won't comment, according to VG247, which signposted the news. There are, therefore, no more details to go on.
Perhaps this is the long-awaited arrival of Robin and a co-operative mode. That would be nice. Or, more realistically, this may be another Challenge Room.