I think this article, from the National Post, provides interesting discussion:
"SYDNEY, Australia [which should be read somewhat wistfully] -Heath Ledger is mesmerizing in The Dark Knight, the latest Batman film. Here in his Australian homeland, his posthumous appearance as the Joker has been a major news story for two weeks.
"It's an extraordinary film, even if you are, inexplicably, unmoved by the addition of futuristic gadgets to the most reliable blockbuster combination in cinema: explosions, firearms, car chases and more explosions. This Batman comes with the bonus of some of the more combustible questions in philosophy. What is evil? Is there a moral order built into our world, or is to speak of such a moral design delusional?
"This Joker does not permit us to dismiss him as delusional; he comes with an argument. This is not the maniacal buffoon of Jack Nicholson's star turn nearly 20 years ago. This Joker is diabolical."