Michael Emerson from Lost is to voice The Joker. Funnily enough, his is a name which keeps coming up on wishlists to play a live-action Riddler. Here's what he has to say about playing the clown prince of crime:
“He’s one of those great villains that an actor like me likes to play because he has layers – he has a face for the world and a face for himself. He is, in a way, a villain who is a natural actor. All of that is that is delightful and confounding at the same time. It’s freeing because of the largeness of it. When he laughs, there’s nothing subdued about it. When he screams in rage, there’s nothing controlled about it. The amp is turned to 11 at all times.”
There's also a pic of what the Joker from TDKR will look like at the above link.