I don't have much to add to the discussion of the death of Heath Ledger beyond what has been said already: a tragic loss of an actor whose best work was ahead of him.
However, some of the news reports are citing the grueling shoot for The Dark Knight, and the resulting exhaustion and sleep deprivation, as a factor. Ledger was apparently taking Ambien (among other things) to help return to regular sleep patterns.
Those reports reminded me of Haskell Wexler's interesting documentary about sleep deprivation and exhaustion in the film industry, Who Needs Sleep? (2006). Wexler, one of the great cinematographers of all time as well as an outstanding filmmaker in his own right (the groundbreaking Medium Cool)and labor activist, made this film after the tragic death of a crew member on the film Pleasantville, who fell asleep while driving home after several days of long shooting schedules. The film documents not only the dangers of sleep deprivation combined with long work days, but also the systemic causes of such onerous shooting schedules and the studios' resistance to changing the habits that lead to these problems.
From an aesthetic perspective, the film is not Wexler's best, and it spends a bit too much time repeatedly explaining the general health problems related to sleep deprivation and the history of labor issues related to long work days. However, it is interesting to see that there is still an industry in America where workers have to fight for a 14 hour work day. It also anticipates that the main way this cause might get attention is through the loss of a celebrity.
If you have Time Warner's OnDemand service, you can catch this documentary for the rest of the month in the Sundance Channel's free movies. The movie also shows up on Sundance from time to time, so you might check it out then as well.