It seems like that expensive surprise is out of the bag, if some inside scoop shared with Hollywood Chicago is to be believed -- but I don't think it's that expensive of a revelation. Hall will play a reporter for the GCN who is really jealous of the lovely Bruce Wayne (Christian Bale). The guy must be one heck of a reporter, or come from money too, because he has the same car as Wayne -- a Lamborghini Murcielago. He finds out that Bruce is Batman and wants to use his work to out the secret (as an antagonist rather than a villain). The Joker (Heath Ledger*) isn't happy about this and sets out to stop him. Really, the source could've said that Hall's character challenges him to a sumo-suit duel and I would be happy -- the film has got a great cast and should be one heck of a ride.
*Name corrected thanks to mbstrlbstr.