There's no debate necessary. When in a theater or concert hall (or at events like weddings and funerals), turn the ringer off. Period. If you are a "must-carry" kind of person (on-call surgeons, for example), then get a phone with vibrate feature -- most phones have this now, some by using an optional battery. On a similar note, heaven help you if you phone goes off on the golf course while I'm in the middle of my backswing... you'll be wearing my club.
As for etiquette, when using your phone in public, remember that you're in public. If the conversation you're having would otherwise be inappropriate for the setting (either in content or volume), then it's inappropriate to have the same converstaion on your cell phone.
Finally, always respect any rules/requests about cell phone use made by the proprietor of the establishment you are in. In some cases (hospitals, for example) it is a safety issue; in others, it is an expected courtesy toward your fellows.
I cannot conjour a more egregious cell phone faux pas than letting it ring during a symphony performance, when sound is what matters most.
And when Mahler's long symphony finally culminated in an evocation of the glorious resurrection a cell phone rang....to the tune of Smoke on the Water.
Though it appears that (mostly younger) people choose to ignore basic rules of courtesy while using cell phones, even these people are not too stupid to know that a ringing cell phone during a public musical performance is not a good thing. We need to understand that it is really we who are at fault because we fail to grasp that anyone who lets their cell phone ring during a concert must be a really, really important person who cannot be bothered with any concern for the rest of the audience or the performers.