This happened to me for the first time recently. I read Radio Free Albemuth by Philip K. Dick and just recently found out the movie is coming out on June 11th. What books have you read without any prior knowledge of a movie existing?
Choke is already completed; it was shown at Sundance. You can read a review of the film here:
Another Philip K. Dick book, and one of my favorites, Do Androids Dream of Electric Sheep. Of course the film adaptation Blade Runner was actually made in 1982, I never saw it until just recently when a friend informed me that it was actually based on a book by one of my favorite authors.