About Heath Ledger playing The Joker, I think he might be one of the better choices to actually be able to pull it off. He definitely is talented enough to play such a role, I think.
About Jack Nicholson in the first Batman movie as The Joker -- loved it. I thought he was great in that role and one of the best villains in the Batman movies, so far.
About Nicholson not happy being in Joker in the next Batman movie, he's probably unhappy he ain't getting a big paycheck in a big mainstream film.
So, I guess we'll just have to wait and see how Ledger's performance in The Dark Knight pans out to see if Jack's right or not.
I do think Nicholson's a great actor and respect his skills -- see The Departed, Batman, The Pledge, and About Schmidt, just to name a few -- but that's as far as that goes. I think he's just running his mouth there.
Personally, while I love Burton and I think Jack is a cool enough guy, I hated every fucking Batman movie ever made until Batman Begins, and that movie is the sole reason that I'm looking forward to the next one. I thought the rest of them were just bloody awful.
I think Batman Begins was the best Batman movie since the first one from Burton.