The Da Vinci Code seems like a natural movie blockbuster, an easy slam dunk. But “Da Vinci, set for release in May, is shaping up as one of the movie world’s more complicated exercises – so much so that Sony has dropped a scrim of secrecy over the affair, refusing to discuss anything but the barest details. The script has been closely controlled. Outsiders have been banned from the set. And those associated with the film have had to sign confidentiality agreements.”