When a young singer dies
To our shock and surprise
In a plane crash or flashy sports car
He becomes quite well known
And the kindness he's shown
Has made more than one post mortem star
—Phantom of the Paradise, "Goodbye, Eddie, Goodbye"
I have to give a lot of credit to the writers, since they've come up with a villain that would give Homeland Security the willies. Ledger's Joker isn't some demented buffoon. He's smart, agile, and fearless. He chafes at a world crawling with phonies and hypocrites. While others succumb to the lure of power, influence, prestige, religion, or money, this Joker rises above all of that. He's a leader born of necessity, because no one else is truly worthy. His voice is often a voice of reason, but it's lost in the noise unless he becomes the center of attention. His flair for the dramatic, plus a total lack of inhibition against provoking chaos, murder, and mayhem, help keep the spotlight on him.
Christian Bale's Bruce Wayne/Batman takes a back seat. He can't quite figure out if he wants to be James Bond or Tony Stark. Either way, he comes across as neutered. Fortunately, this isn't his story: he's just the glue that binds everything else together.
So yes, Ledger's the star, and I expect he'll get enough sympathy votes from the Academy for a posthumous award. As an ensemble work, it's a contender for best picture, although I'm disappointed by some of the casting. Nestor Carbonell's Anthony Garcia is too much like Lost's Richard Alpert, and it's hard to believe the mayor of a major American city wears goth eyeliner. I also found it hard to accept Maggie Gyllenhaal as Rachel Dawes; she really seemed to be channeling Kirsten Dunst.