When David Niven died, he had “…just completed work on Trail of the Pink Panther and Curse of the Pink Panther.
He was incomprehensible at times during the filming of both movies, (due to his advanced ALS), and
his voice was dubbed over in post-production by impressionist Rich Little, a fact that Niven later learned through a gossip column.” (ed. note: source: Wikipedia)

same prospect now faces the producers, directors, and editors of the
mostly-finished, but unreleased movie: THE DARK KNIGHT.  Heath Ledger,
the male lead in the film, had completed most all of his on-camera
appearances, but — as if often the case — some post-production with
his voice in ADR (automated dialogue replacement) was likely needed. 


I suppose Rich Little would still be available, but unlikely.

more about this fascinating conundrum in this article by Steve Daly.
It reveals a lot of the inside workings of the movie biz, and
opportunities for REALLY GOOD voice-actors.