Well, technically, as long as the actual movie doesn't start until after midnight, they are safe. With previews and other padding, a theater could fudge anywhere from 15 to 30 minutes. I know I've waited through about 20 - 25 minutes of previews before at a movie. And with all of the new previews making their debuts on the 18th, I expect a long preview time for Two Towers as well.
Another reason I like smaller theaters. Last year I had to suffer through only one trailer.