Tell-tale signs perhaps. I would like to think this was an accidental overdose, but details are still sketchy. Truth be told no one will ever know. Word is playing The Joker physically and mentally taxed Ledger.

To prepare for the role he lived alone in a hotel room for a month, formulating the character's posture, voice and psychology. He started a diary, in which he wrote the Joker's thoughts and feelings to guide himself during his performance. [source]

Not to be overly dramatic, but this really sucks.