Why are people bothered if The Dark Knight overtakes The Godfather and Shawshank as the top film of all time on IMDB? Does IMDB's list have to 100% represent your taste? Does Wall.E deserve its 25th ranking (that's 156 places above Toy Story)? There are 3 films in the top 10 that I wouldn't even consider having in my own top ten, but who cares?
Edit: And about the Ledger issue... people WERE excited about his performance before he died. I don't believe any of the hype over his performance was contributed to by his death - it's an amazing performance, immersion in an acting role we haven't seen for a few years.
No disrespect intended to other actors for the above comment, but Ledger did go a step further. Compare it with the performance of Leonardo DiCaprio in Blood Diamond. Decent film, and DiCaprio is a good actor, and DiCaprio got a Best Actor nomination.
Have more people seen the movie because of his death? Maybe, to see the last performance of a talented actor, but I believe the ratings people are giving it are genuine reflections of how they feel about the film. More people seeing it out of curiosity = more favourable ratings, because it's the kind of film that'll get more favourable reviews than none favourable, from the average filmgoer. That has nothing to do with Ledger's death, but everything to do with his performance and the other actors and the movie itself.