Exactly. The whole "everything in this book is fact"....or something to that effect...that he has in the beginning of the book is to put you in the mood. I loved reading the book and reading things I just knew were not true but in the world of the novel, having these things as fact makes everything around them that's happening much more interesting.
Michael Crichton does this in his book as well. For instance, in "Eaters of the Dead/ The 13th Warrior" the introduction says that the book was an exact translation of a recently-found manuscript of a viking fellow. Or in Jurassic Park's intro, it talks about the corporation InGen as if it actually existed. Although I will say, they were a bit more tongue-in-cheek than Dan Brown's intro, where he pretty much says "The following book is based on fact." Brown needs to learn that subtlety is a good thing.
If so many other books have done this in the past, I wonder why "Da Vinci Code" got so many people so worked up? Is it because it's about Jesus?
I personally thought the "Da Vinci Code" movie sucked, though.