The race for the Palme d'Or in Cannes has been thrown open again with a mighty challenger coming up on the inside rail. Julian Schnabel's "The Diving Bell and the Butterfly" is a wonderfully acted and profoundly moving adaptation of the memoir by French magazine editor Jean- Dominique Bauby, who, at 42, and at the height of his brilliant media career, became paralysed with a stroke. Peter Bradshaw reports.