Specifically, the game will release in 2009, is much darker in tone, and is being written by Paul Dini, author of Batman: The Animated Series. What's more, the game is said to rely on both detective and combat elements, will run on Unreal Engine 3 (most likely confirming the title for at least Xbox 360, PS3, and PC), and is played with an over-the-shoulder perspective.
Starring villains so far include Joker, Harley Quinn, Killer Croc, Zsasz, Penguin, Riddler, Scarecrow, Poison Ivy, and Mr. Freeze.
It's unclear whether the Arkham Asylum announcement will have any effect on the still unreleased Batman: The Dark Knight, which was confirmed in development last year, before recently dropping off the radar.