This is an old chestnut in PPRuNe and has been discussed many times in the short time that I have been here. Firstly, I am in telecommunications and have been so for 22 years.
Jamming of Signals: Equipment available now. Being considered by libraries, theatres etc. Objections likely to bew that many folks do keep their phone on silent so that they can receive notification of calls via SMS. Doctors on call out etc. Problem in the cabin is, of course, possible interfence of valid signals.
Without looking at the specification of the jamming device, and reference to each AND EVERY airport and airline, it is not possible to comment. My guess is that the chance of it happening is zero. Carriers will say that it is the technical problems but actually it is the cost. Across a fleet it would be VERY expensive - not so much the actual box/es of tricks but the certification that it does not interfere with valid signals.
The same goes for a detector on the air bridge. It is not possible to detect a piece of electronics that is switched off, so it would only find live units. The cost would be prohibitive and the airports would say that it is the airlines problem!!
Possible solutions?
On the air bridge, have automated speaker announcements. If the airbridges already have speakers for emergency use, then the lost is low, perhaps one thousand pounds per bridge. If they do not have speakers, the cost jumps and the airports will not do it. Bear in mind that BAA is the most parsimonious of companies and have a stranglehold on the UK and influence elsewhere in the world.
In the cabin:
As soon as Pax start loading, automated announcement:
Please switch off all mobile telephony equipment. Use of any such equipment endangers this aircraft and all it's passengers and crew.
This will enable (empower) pax to tell their neighbour to switch it off. The announcement gets repeated until doors closed and then repeated in safety show.
Upon arrival, it must be played during final approach and taxi. The announcement must be made in several languages, particularly the ones of countries visited on the flight. At a guess, eight to ten languages which will help to relieve the monotony of hearing the same thing over again in your own language. If the a/c has TV monitors then it can be shown on them as well.
Cost? When compared to providing the latest news bulletin every day on long haul? Practically zero. Can be implemented without any legislation etc.
I agree with missioncontrol that a major prang has to happen before anyone 'in authority' does anything. I have said so myself every time this subject comes up for discussion. After all, no one will agree to enforce (not legislate but enforce) a ban on the use of mobile/cell telephones for those driving cars, where the danger is more immediate and more evidence exists!