I can be reasonable to assume (since this whole post is all assumption and speculation) that Ledger's death added to the box office that first week. Ledger's alive, it makes a shitload of moneyl. He's dead, it breaks first-week records.

After that? I don't think so. This movie is getting second, third, and fourth viewings. You don't get that from customers who wouldn't have seen the movie without Ledger dying. That's hardcore fans that loved the movie.