A video interview on GameTrailers from PAX 2011 reveals a "new game plus" mode for Batman: Arkham City, which will offer a different gameplay experience during a second playthrough of the stealth/action sequel (thanks nin via Shacknews). Here's the explanation from Rocksteady's very enthusiastic Dax Ginn:

So when you finish the main game mode, you can then restart that, with all of the gadgets and all the upgrades you got in that first playthrough, but you're going to need them in the second playthrough, because all of the enemy configurations have changed, all of the assistance you get in combat with those blue lightning bolts coming up against guys who are about to hit you... They're gone, so you have to read their animations coming in. So that's why we really wanted to drive home the replayability with Batman: Arkham City: Taking those gadgets back out on the streets, delivering justice a second time around, but really asking you, "Are you the Dark Knight? Have you got what it takes?"