Seems Mr.Ledger had more in his bloodstream than just benzodiazepines.   There was also oxycodone, hydrocodone...which is more likely to have killed him.   Benzos don't do that, the lethal dose of benzodiazepines is just so astronomical, that unless combined with other medications,they don't usually kill people.   The narcotics, on the other hand can be lethal, since at high enough dose they shut off respiration.  If he also had pneumonia (which is suspected) that would make him more susceptible to the respiratory failure. 

Maybe it wasn't "suicide" in the traditional sense, but really, it was certainly very reckless behavior on his part taking all of that combination.   Maybe he did it gradually over days, and it caught up with him.  

http://movies.yahoo.com/mv/news/ap/20080206/120232362000.html