I my opinion, the the critics, conservatives and butt plugs were all used as tools of the promotion of the movie. Not much direct marketing, but tons and tons of "oh no it is against god" rhetoric on TV news and in the papers and whatnot. All of these people saying "no no" pushed everyone to see it - same basic marketing as porn, if you think about it.
You can read more of the hype here:
http://latestmovies.com/search/da%20vinci
Hype is a very powerful way to get people to pay attention... between this and Passion of the Christ, you can see an entire marketing plan played out.
Alex