I think the Joker stuff will be available as DLC on 360 eventually, in a similar way to Soul Calibur, 360 had Yoda & PS3 had Vader, eventually they became available to both consoles.
Might depend on sales, if the 360 version shifts more units, as is normally the case for multi-platform releases, the developers might see sense & let the majority have the Joker modes, just a thought.