It appears as though the PS3 version of Batman: Arkham Asylum will be getting exclusive levels that allow you to play as The Joker.Eidos, the publishers of the game, have in fact confirmed the proclamation that PS3-exclusive content will contain Challenge Maps - levels in the game that you can attempt multiple times in order to complete and get the best score – that feature The Joker as a playable character.
These levels will be available as downloadable content.
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