There is a cheat that was recently release that allows you to choose a different costume for Batman before completing the campaign on normal or harder, it can also be used after completing the game on easy.
You must enter this cheat after you have chosen your save file when you are at the main menu, and if you quit the game and want to choose a different skin for batman you must enter the cheat again.
Left, Left, Down, Down, Left, Left, Right, Up, Up, Down.
You should hear a confirmation sound if you entered it right. Then just select continue game and before you come to the loading screen you'll be allowed to choose the skins you have unlocked or bought. Also, this exact code works on PS3.
Added by: Keyblade21 Dec 29th 2011, ID#8095