this doesn't really count but i read the green mile series when i was pretty young and said that of all his books, this is one that should be made into a movie... and then a couple years later they did but i still haven't seen it.
i am ashamed to say that i didn't read harry potter until after i saw the first movie. now it is my most beloved book, i reread all the time, especially when i'm going through a rough time cuz it is always good and within a page i am absorbed and forget anything that is going on in my real life.
with lotr, i saw the first movie and HATED it. then i read the hobbit (possibly not until after the second movie). once i read the hobbit, i liked the movies a lot a lot. i didn't read the trilogy until about a year and a half ago and i'm glad i saw the movies first because i love the books and would have been annoyed with some of the discrepencies if the movie hadn't been what i came to know originally.
i'm listening to the da vinci code on tape now, though i wanted to read it before it became a movie i am just now getting around to it. i didn't love the movie but i'm liking listening to the audio book a lot.
i just read inkheart and inkspell and shortly after finishing them i learned that inkheart is now in preproduction to become a movie.