I don't see much point in hoping for or being saddened by Oscar results. For all the excitement, they're just a poll of people's opinions, and aside from a few truly awful examples, art, especially film, is pretty subjective. I don't think TDK is worth an Oscar, it was pretty common place superhero-action movie stuff, and disappointing compared to the hype it generated.
If Ledger doesn't win, it's a criminal act of ignorant stupidity. His performance was one of the best of all time, of any film. He truly made the character live. You don't see Heath Ledger in that film one bit.
That's what's supposed to happen with acting, aside from some of the biggest stars who can get away with playing similar characters. I think Hopkins in TSotL, Nicholson in A Few Good Men, the leads in The Last King of Scotland gave much better performances. I admit, part of why I wasn't impressed with TDK's Joker was the writing and use of him, but for me it's hard to see the performance aside from that.