WB Games has announced that the official name of the Batman: Arkham Asylum sequel will be called Batman: Arkham City, which will be released in the Fall of 2011.
Not much else is known about Arkham City, besides that the maximum security prison is now located within the Gotham City city limits.
WB Games has also opened up batmanarkhamcity.com, which has the trailer and the ability to sign up for updates emailed directly to you.