The Worst Nightmare Batman Skin has been removed from all versions of the game. For PS3 and Xbox 360 players, when completing I Am The Night Mode, players are awarded the Waynetech Theme for PS3 and the Batman Avatar on Xbox 360. PC and Wii U players do not unlock anything upon completion of I Am The Night Mode.
It’s currently unclear why this change has been made or if the skin will be made available in another form at a later date.
Source: WB Games Support Forums