Ledger should win something more than an mtv movie award (which you know he'll sweep cause mtv is too unoriginal). Maybe not an oscar cause its not really your typical, conservative oscar role, but he will get a nod for it.
The Dark knight was also considerably cheesier than Batman Begins (The cell phone SONAR eyes, that was too much), so I wouldn't be suprised if they got someone else for the joker. People can always recreate a character, but at least people won't forget who created it.