See the New York Times article reporting it.

I’d rather not comment on the circumstances (strange as they seem), but I can say I will really miss Heath Ledger. I remember seeing him in A Knight’s Tale for the first time, and most recently in Brokeback Mountain and I was waiting on the edge of my seat to see him in The Dark Knight (which IMDB says is in post-production now). He was a great actor whom I thought was really on his way to the top, with amazing skills, and I’m really going to miss him.