I read something written by a commercial pilot that as soon as they are on the ground they hear a mass of pings as cell phones are switched on and look for a signal. I don't know if this is true. (maybe jumbodriver can confirm this or scotch the rumor) But if it is true, ignoring the instruction not to use a mobile phone during taxiing could be very dangerous. It would only take the pilot to mishear one word in the communication with the tower and the results could be catastrophic. Remember, the worst ever air disaster took place on the ground.