Read The Da Vinci Code now, before May 2006 when the film version is released. The first full-length trailer is out, and it looks good. I've accepted Tom Hanks as Langdon, but I'm glad I read the book before he was cast. Paul Bettany reteams with director Ron Howard, and looks great as the creepy, religious zealout, self-flagellator, Silas. It's a fast-paced, fascinating premise, so give yourself the chance to enjoy it on paper before seeing the movie!
Incidentally, I'm of the opinion that Dan Brown's second novel, Angels & Demons, has everything that makes The Da Vinci Code hard to put down, but to a greater degree. So ideally, you should read that one first.