Batman: Arkham Asylum was a hit when it first released. It was the first proper licensed super hero video game that was actually fun. A sequel, Batman: Arkham City was confirmed to be in the works just recently and apparently, it will feature multiplayer. A component that was not seen in the original.
According to a post on ArkhamCity.co.uk, the confirmation comes via this month's issue of UK magazine PSM3 where Rocksteady have claimed that "multiplayer is a certainty, but no details on this yet."
Batman: Arkham City features a slew of Batman veterans including Catwoman, perhaps some co-op action is in order?