Interesting replies; thanks for that explaination, Fargoo. The electric noise from a mobile is an obvious indication of possible interference (i was woken up the other night by the noise through my radio alarm clock (which was off) from the phone sitting next to it on the bed side cabinet. A light aircaft engineer told me that he never believed that they could affect the systems until flying a C182 with a digital autopilot, the aircraft turned (on AP) to the side the mobile was on!
I understand one reason it can be a particular problem with airliners (as apposed to, say, one phone in a light aircraft) is that over 100 phones all trying on maximum power to find a station would add up to a lot of radio power.