Yes, I think that would be a good option. When you enter the theater, it pops up with a dialog - "You have entered Regal Cinemas - your phone is going to theate

Yes, I think that would be a good option. When you enter the theater, it pops up with a dialog - "You have entered Regal Cinemas - your phone is going to theater mode. Press here to cancel." Making it opt-out rather than opt-in would create much more social pressure to not text during the movie.