Just going to throw my two cents worth in on this discussion.
Yes the book is presented as fiction, but Dan Brown does not do enough to relay that message to the masses. He allows people to think that it has an underlying truth so he can make more money.
The book/movie is Blasphemy any way you look at it, even if it is presented as fiction, it is calling Our Lord a liar and a deceiver. It is bringing into question the accuracy of the Holy Scriptures.
As Bible believing Christians we can see through this, however for non believers and new Christians the deception, presented as fiction or not, is used by the enemy to shake their faith. It plants seeds of doubt in their carnal mind. Books like this and movies like Zeitgeist are but out there as a Spiritual weapon, plain and simply. It doesn't matter that the things in these types of movies are debunked and refuted, the seeds of doubt are planted in the carnal world.
It is our duty as Christians to defend the faith, standing by and allowing these deceptions to occur with our mouth shut is reckless.