Recently, I sat and watched the Back To The Future films.
Each film runs to the standard of that time, which was about ninety minutes. The first film is a self contained story using the classic three act structure, however, parts one & two are basically one film split in two halves, each half running at three acts
If Back To The Future were to be made today, the second two films I think would actually be made and screened as one longer film, which would run at five acts (one act from each film would be fused together, making five and not six acts in order for the story to build more progressively and for the acts to work off each other.)
I really do think that when we hit the cinema these days, we expect a minimum of two hours. So, longer films, means more acts.
I think one of the reasons that The Fantastic Four didn't do so well was because it ran at ninety minutes and had only three acts... Therefore the story was perceived to be too simplistic.
Is the three act structure dead, or at least dying? Is it too simplistic for the story savvy public of today?
I think Greg makes an interesting point. The audience is getting wise to ALL narrative techniques. Its up to us, as writers, to keep one step ahead. It may be the number of the acts, or may be something else, but these days you need something fresh.