Pretty much everyone that played the first Batman: Arkham Asylum game really dug it. It's pretty much a no-brainer that a sequel has to be made. At the beginning of the 2009 Spike VGAs, a teaser trailer was shown for the sequel.
You'll notice that the Joker appears a little under the weather. That can't be a good sign. I know when I get sick, I'm a little cranky. Try to imagine a cranky Joker...
A release date or the platforms it'll be on hasn't been announced. I think it's a good move not to have a firm release date since the first game had some delays. I say it's better to have delays and end up with a better game. There is a website that currently doesn't have much information, www.ArkhamHasMoved.com.
(Yes, I'm aware NightFang posted this trailer in his blog. I wanted to make sure it was seen in the news). Did you play the first game? Who would you like to see pop up in the sequel?
Update: I'm "borrowing" this video from GiantBomb to play it on our player.