Batman: Arkham City fans can download a free DLC costume, namely the Batman Incorporated outfit, tomorrow for free.
Rocksteady have also added a cheat to allow players to access costumes at any point.
Previously you had to complete the game to equip a new tights-and-codpiece combo but you can now press the following combination on the Main Menu screen (after you have selected your save slot):
– Left, Left, Down, Down, Left, Left, Right, Up, Up, Down.
Source: Batman: Arkham City Blog