From what the London papers were saying, playing the Joker might have had a hand in it. App. he had shut himself up in a hotel room to practice the role of a pyschopath and said in a few interviews that after they finished filming, he had trouble sleeping for more then two hours a night.
Jack Nickolson was in Piccidally Circus last night and was quoted by the papers as saying 'I had warned him'.
Regardless, it seems to have been an accidental overdose of sleeping pills and a terrible accident. The man definatly did have talent, its a shame he won't be around to share it more with us.