I was led to believe, and I must say that this seems the best reason for not being aloowed to use a mobile on board, especially after seeing countless people at petrol pumps, and of course the odd one on board using a phone is that...
If people had mobile phones switched on; on board... the phone networks would crash as they could not handle switching between transmitters of that number of phones.
Think about it. Say 150 people per aircraft have phone switched on (as an average guess) and the number of aircraft movements over the UK per day.
It's not like a car where it would be a new transmitter every few minutes, in a plane it would be every few seconds.
I think the base stations they would like to fit lower the switching by factors of thousands, as each aircraft has to switch every few seconds, not the entire passenger load.
If mobiles were that bad they would surely stop cars and things, the amount of electrics in them today.
For the record I am not a fan of phones on planes... I hope that if they do allow them they provide an area to use them.
Maybe a sound proof box on the wing?