Johnny Depp will replace Heath Ledger in The Imaginarium of Doctor Parnassus, the latest project Heath had been working on. Director Terry Gilliman hand picked Johnny for the role.
But this doesn't mean we won't see Heath at all. The studio is saying there's a part of the movie in which Heath's character falls through a magic mirror, and they may have Johnny become the new appearance of the changed character.
It's sad that they already have to be dealing with this so soon after Heath's death, but it's great that such an amazing actor as Johnny Depp will be on board.