The playable Joker will be exclusive to the PS3 version of Batman: Arkham Asylum (PS3, Xbox 360, PC). This is to finally put an end to those rumors claiming that the Joker DLC is also coming to Xbox Live. Rocksteady's Sefton Hill has made the confirmation and Xbox 360 owners will miss out on the must-have add-on.
The Joker DLC came with an exclusivity deal with Sony so there's no way it could be on XBL, regardless of what the rumors may say. Small consolation for Xbox users though, future DLC coming to Arkham will not be PS3-exclusive.
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