I am completely with Mister Matt and the like on this one.
There is literally NO reason why anyone's phone should go off in a theatre, ever. Nor should it be on vibrate. If there is a situation that cannot wait for the 2ish hours you are there, then I'm sorry, but maybe you should not have gone to the theatre in the first place. There are always family emergencies and stuff like that, and they are always unfortunately timed and always crappy. But to leave a phone on in the off chance that something might happen is not fair to the others who will inevitably hear the text messages and such that go along with leaving your phone on.
The part that always gets me is during the Pre show announcement, where people are warned to turn off their phones, etc and unwrap candy, and they always laugh, as if to say, "Haha obviously." And yet, they do it anyways.
I am extremely anti-cellphone-in-theatre. Maybe I should consider changing my profession to usher....I think I would be good.
"You see, I told you so! There's lots of things I know. 'Ponine, she knows her way around."