Heath Ledger is not as emotionally challenged as his Joker character so we can assume this is not an attempt on his own life.
Gossip site JustJared reports:
Heath Ledger was spotted shooting scenes for his upcoming fantasy film The Imaginarium of Doctor Parnassus in London Monday night.
In this scene, Heath was seen hanging from London’s Lambeth Bridge and was rescued by costar Andrew Garfield.
So Ledger appears to be taking on roles that are of some pretty wacked out characters. First the Joker, then of this odd fella who appears to have hung himself. He even did his own stunts which lead him to this predicament.
I am assured that this is not a spoiler as he is supposed to be part of some sort of performance art group that stages these outrageous events.
JustJared has more images of Heath on set.