I read a report about two years ago relating to a helicopter which experienced a cargo fire warning, fortunately while still on the ground. An active mobile phone was found in the baggage, switched off and the flight continued without incident.
After September 11th, I would think that any inflight emergency would result in many passengers picking up their mobiles and trying to say goodbye, particularly in the caser of cabin crew with marginal english describing bomb threats!
I agree that a ban would be the only answer.
ATB, PTC