According to "Bags n' Boards," the comic industry blog over at Variety, Heath Ledger has been cast as the Joker in the next Batman film. The second Batman film will have the same creative team as Batman Begins, Chris Nolan at the helm and David Goyer (of JSA fame) with co-screenwriting duties.
Tom McLean at "Bags n' Boards" has a decent discussion of the history of the Joker role as played by Cesar Romero, Jack Nicholson, and Mark Hamill. He even adds some speculation with how certain spoof trailers might show up on the internet with references to Ledger's performance in Brokeback Mountain. I, for one, seem to remember that Batman and Bane had a Brokeback moment and that the Joker's Brokeback moment was with Batgirl (Barbara Gordon) in Killing Joke, but that is just me.
As for my opinion regarding the casting, I think that it is a spot on selection. I never really liked Nicholson as the Joker. First, he was over the top Jack and not over the top Joker in his performance. Second, as much stage presence as Nicholson has, he wasn't angular or "striking" enough to play the Joker. His Joker looked like a silly clown. When the first Tim Burton Batman announced that Michael Keaton would star, I hoped that he would be the Joker. If you combine his character in Beetlejuice with his performance in Pacific Heights, I believe you capture the Joker's personality to a tee. Besides, Keaton's angular eye brows and curly hair almost make him look like the Joker without the makeup. For me the quintessential drawings of the Joker are the Bolland stuff from Killing Joke , he just seemed to capture the insanity perfectly.
I look forward to seeing what Ledger will do with the role. He has shown he can do action, and that he can act, but can he pull of sympathetically crazy? That's what I think Nicholson's Joker lacked, a sympathetic-ness to which the audience can be horrifically drawn. Goyer has a great knowledge of the DC Universe and has revitalized many of the historic characters (as a comic book writer) in his work in the JSA title. He has made silly villains seem ominous (Kobra anyone?), so I have high hopes for the next Batman movie.