If a group of people are gathered for an event, using a cell phone should be forbidden. This includes all ceremonies, concerts, theatre, etc. After suffering through someone being on her cell phone consistantly during last year’s Broadway on Broadway concert (Hi! Guess where I am? You know who’s singing now?), I made an annoucement at this year’s that nobody should use a cell phone so we can actually hear the music. And nobody did.
In a grocery store and other non-eventful places, it doesn’t bother me. People should speak in a normal tone, and hang up at check-out lines or any other place of human interaction. And NOTHING is ruder than being on the phone with someone who says “Hold on a minute. My cell’s ringing” and leaves you hanging and listening to their other conversation. I hang up on them.