I came to appreciate cell phones this past week. I used mine to coordinate pick ups at the airport, to arrange for meetings, to check on items that needed to be purchased, etc. I did NOT appreciate cell phones when they interrupted the interaction between people in a group. I received a phone call when we were at a dinner. It was a message I had been anticipating. I took the call, received the information, said I was with other people and could have to get back to the caller later. To me, that's a reasonable use of a cell phone.
Conversely, there was more than one occasion when our group interaction was interrupted by a phone call from friend of someone at home, who was calling to chat. The receiver of the call then essentially turned the rest of the group off and continued to chat--loudly (I think we tend to speak more loudly on a cell phone than we realize) the the caller, who obviously was more important then the people in the group. In one instance, the member of our group receiving the call spoke all through a store, and while we were walking around admiring the scenery. There was no suggestion that the call would be returned at a more convenient (i.e., less insulting) time; instead we were made blatantly aware that whoever was on the other end of the line was so much more interesting than we were, that we could not only be ignored, but our own conversation could be hampered because we could not speak over the level of the conversation going on on the cell phone.
On a lengthy automobile trip, a cell phone arrived and all music, conversation, and anything else in the car came to a hault while the person on the cell phone carried on a chat with the person on the other end.
I think there is definitely a time and a place for cell phones. I can even see where receiving a call while you are in a social setting can be excusable. But when that call becomes the focus of your attention, to the exclusion of the people with whom you are visiting face-to-face, I think that's just plain rude.