"Bat Theft Auto" Pandemic title may still prove true, we now have definite news of a new Batman game in the works care of Game Informer. Batman: Arkham Asylum is being developed by Rocksteady Studios (Urban Chaos Riot Response) and Eidos. According to Game Informer's online teaser, the Joker will be the main villain in the story of a prison break gone awry, written by Paul Dini (Justice League, Batman Beyond).
Additionally, a 10-digit countdown can be seen on Rocksteady's site. Our math is a little fuzzy, but it's going to take approximately "a good while" (i.e. over 2 years) to reach zero.
Update: Some details reportedly from GI have been posted on NeoGAF (read: we can't confirm any of this at the moment). The Unreal Engine 3-powered game is going for a gritty, Chronicles of Riddick-esque feel. DC Comics' Wildstorm is crafting the characters' look. Some villains in the game include Joker, Harley Quinn, Killer Croc, Penguin, Riddler, Scarecrow and more. According to the post, Arkham Asylum is projected for a 2009 release on PC, Xbox 360 and PS3 and is not being rushed to tie into any TV or film marketing cycles.