Strictly a SINGLE PLAYER ONLY experience:
http://uk.ps3.ign.com/articles/114/1149377p1.html
Awesome news. I don't need multiplayer in this game.
IGN: Now, give us a Valentine's Day gift and tell us SOMETHING about multiplayer. You know that the couple that plays together stays together and that works for friends or significant others. What's going on with multiple players in Arkham City? Co-op? competitive? We must know!
Sefton Hill: There have been a number of rumors circulating about a multiplayer mode in Batman: Arkham City so let me start by saying, once and for all, that Batman: Arkham City is a "single-player only" experience.
http://uk.ps3.ign.com/articles/114/1149377p1.html
Awesome news. I don't need multiplayer in this game.