I would suggest that a better approach would be to standardize a cell-phone "courtesy" signal which, unless overridden, would cause suitably-equipped cell phones to disable the audible ringer while keeping other features (vibrate, messaging, etc.) active.

Additionally, I would like cell phones to provide a button which would answer the call with a message that the holder of the phone would be taking the call shortly. This would allow someone who was e.g. in a movie theater when the baby-sitter called to discretely leave the auditorium before having to take the call [anyone who thought they might have to take a call would, of course, be asked to seat themselves along the aisles near the back of the theater].

These features could thus avoid much of the 'social annoyance' associated with cell-phones without reducing their usefulness for emergencies, family and otherwise.