I'm pretty certain that if Ledger does win the Oscar, it will be because the Academy felt he deserved it. There haven't been many post-mortem winners, but there have been a few post-mortem nominations, and most of them don't actually win.
One last thought to add on to what I've already said, I think that some pretty big credit should be given to the re-creation of the Joker character as we know it. We've always seen a certain portrayal of his character, with some noticeable differences, but certainly not nearly as subversive as we've seen in The Dark Knight. Ledger redefined the way we will think of The Joker from now on, but never truly before. Even having read The Killing Joke and a few other graphic novels of a dark nature, this was a performance that took me by surprise in his interpretation of the role.
One last thought to add on to what I've already said, I think that some pretty big credit should be given to the re-creation of the Joker character as we know it. We've always seen a certain portrayal of his character, with some noticeable differences, but certainly not nearly as subversive as we've seen in The Dark Knight. Ledger redefined the way we will think of The Joker from now on, but never truly before. Even having read The Killing Joke and a few other graphic novels of a dark nature, this was a performance that took me by surprise in his interpretation of the role.