While I think Ledger's Joker is receiving a lot of hype, for a couple of reasons - of course his death, but before that because people are rather amazed that anyone would follow Jack Nicholson - though that baffles me since his Joker was pretty weak IMHO. It was the exact opposite of what Ledger's performance is being reported as. Ledger supposedly disappears into the role, which an actor should do with something as elemental as Joker, whereas in Batman '89 it wasn't Joker but Jack Nicholson as the Joker
However, the reviews I've seen from respected critics have made plenty enough reference to Bruce Wayne's development for me to have few fears that the movie will actually be about Batman. Every interview I've read from Christopher Nolan and Christian Bale are all about what happens to Batman as a character, for which Joker is a means to an end. Recall the hype of Ra's al Ghul for Batman Begins. And that movie was completely about Batman.
Ledger's tragic death is going to continue to draw media attention. Who cares? The media isn't the movie.