I thought the movie was post production, isn't it still coming out?
He meant in a different movie.
They won't be using the joker for a long, long time (hopefully) so there's no worrying about Heath Ledger not playing him again in the next movie.
edit: also, whoever jumped all over psyburn up there, shut the hell up. You read far too much into his statement. He wasn't upset because he didn't get to sell his autograph, he was just stating that other people were cashing in.