I'm a Catholic and I could care less about a boycott, I'm probably not going to the movie because:
1) I'd much rather save my $9 for X-Men 3 the next weekend.
2) The book was totally overrated and the movie is clearly a cash-in on it's absurd popularity, even Angels & Demons was a bit better, they should at least make that into a film first.
3) The book does alot of somewhat lengthy explaining things, which only on rare occasions has proven to translate well to the big screen.
4) I don't care what the UK courts say, Brown's "research" consisted of reading other authors' works and then changing it around a little.
5) That mullet-ish type haircut makes tom Hanks look like a tool.
As for it angering people, I'd say it depends on the person. Obviously the religious group the book is less than kind about is going to speak out against it. Lots of relgions/groups have called for boycotts against other fictional writings, shows, films, etc. and we don't make threads about it everytime. Some people like the Opus Dei folk I think have a right to be a little pissed off because they are portrayed totally inaccurately in the book and now due to it's popularity their are no doubt looked at funny for that same false portrayal. Also, on rare occasion I love running into people who take the Da Vinci code as seriously as a lot people do the bible. In short, a few do in fact believe the whole thing is so possible that to them it isn't just another "murder, mystery, conspiracy book", it's non-fiction.
I think the bottomline is yes, it is fiction (average fiction at that), but since when has that stopped anyone from getting mad about anything. People always find a reason to get their panties in bunch over something. We live in a world where at least to some, fictional books can't be just that, South Park can't just be a comedy show, drawings of Mohammed can't be cartoon strips, comedians can make racial jokes but only if they are in that ethnic group, etc.