Joker Cast for "The Dark Knight"

The sequel to Batman Begins is being called "The Dark Knight", and Heath Ledger has been cast as the Joker. Hmmm. The only thing I've ever seen him in is 10 Things I Hate About You, in which he had the task of taming the shrew. And, yes, this is the same Heath Ledger who was nominated for an Oscar for his role in Brokeback Mountain. He's not an obvious choice for the role, but I'm not entirely against it. There's potential. But what I really want to know is, can he laugh?