The theory that the Holy Grail is actually the blood-line of Jesus has been around for some time. Baigent and Leigh had a best-seller in the 1980s with "The Holy Blood and The Holy Grail". They do make a convincing and well-documented case, receiving wide acclaim, although the ecclesiastical authorities were unimpressed and damning in their criticism.
After reading "The Davinci Code" I was interested to catch up on Dan Browns' other works but found them fairly lightweight and definately not in the same league.

C-flat.