Every movie theater I know of in my town to has signage out the front banning you from using cellphones inside the theater. And at the theater I actually go to (because its the cheapest) they have a big message on the screen at the start of the film saying "please turn your cell phone off". I never experience problems with cellphone usage at this particular theater either (most people seem to respect the signs). Other theaters I have visited in the past also have the "no cell phones" thing on the screen before the film.
Why dont more theaters in the USA do the same thing and simply tell people to turn their phone off in the theater? If even one person chooses to follow the sign and turn off their phone when they wouldn't otherwise do so, its a win for all the people who just want to enjoy the movie. And its not like someone is going to not go to the movies just because they are asked to turn off their phone (if you need the phone active so you can be contacted in an emergency, keep it on silent/vibrate and step out of the theater to use it)