Just in from /Film [1], which is citing AICN [2]:

All of the rumors, concerns and speculation regarding how Heath Ledger’s unfortunate passing would be addressed in director Terry Gilliam’s The Imaginarium of Doctor Parnassus, in which Ledger had a major role, can now cease. AICN is reporting that three actors will pay tribute by filling in the remainder of Ledger’s performance (to me, this conjures Bob Dylan and I’m Not There), and those actors are…

Johnny Depp

Jude Law

Colin Farrell

In the film, Ledger’s character reportedly falls through a “magical mirror” into an alternate reality, and thus the caveat of these three actors’ interpretations is further supported. But even sans that plot point, this could turn out to be a quite inspiring and tasteful send off to the young actor.