Brad Pitt is involved in all kinds of switcheroos recently. A few weeks back he left State of Play, to be replaced by Russell Crowe, and now he himself is in talks to replace Heath Ledger.
Pitt is talking with Terrence Malick about taking the lead in Tree of Life, the director's next movie, which is currently without a definite plot because, well, this is Terrence Malick we're talking about and he doesn't do easily explained plots. Ledger had been set to star with Sean Penn. There's no reason given for Ledger's departure. He's currently filming Terry Gilliam's The Imaginarium of Doctor Parnassus, but production on Tree of Life isn't set to start until spring, when Ledger would so far seem to be free. Penn is expected to stay in his role.
Ledger's stock is rising by the second in the film world, so it's a shame to see him go as we expect he could have done great things with Malick. But Malick's movies are always very beautiful, so it makes sense that very beautiful people, like Pitt, should star in them.