In a recent interview on Fox, and I believe on Larry King Live as well, Eric Roberts gave us a little info on a particular scene in TDK.
He said that Joker is blackmailing Maroni's mob for some reason - he has quite a large speech, according to Roberts.
This may tie in to my suspicions for the reason that Joker robs the mob bank in particular in the beginning of the movie - I think that the blackmailing scene works almost directly into this event although it will most likely not occur immediately after.
It would seem that Joker wasn't stealing the money just to do it but that it served a purpose in getting something else accomplished - he makes the mob do something.
We can only speculate what that something is at this point.
EDIT: here's the video from Fox:
[YT]JXHmaH4SWJg[/YT]
--Cameron