Actually the issue is that certain people including Dan Brown are going around drumming up publicity by saying it is all based on truth and that as an art historian he did incredibly detailed research to make sure. I did a review of the book here -
Lisa's Da Vinci Code Pages and I had to do an entire page on why people spend so much time picking at the details. It's because Dan spends so much time claiming they ARE true - which is why the book is selling so well. If he just said "it's all made up" I imagine it would have faded away a while ago, because like you all said, much of this is very well known information.