When passengers are asked to switch off their mobile phones because "they could interfere with aircraft navigation systems" there obviously is some basis to it.
Aircraft etiquette maybe one thing, but aircraft safety is altogether another matter...
Mobile devices can as rudderrudderrat has said, interfere with any aircraft system that uses any type of transmitting signal.
A very disturbing thing I have noticed is that passengers keep their cell phones switched off during flight but start switching them on during final approach. The attitude is: 'well, we're landing anyway'. This may pose a particular threat if an aircraft is on ILS autoland. Since ILS uses microwaves (same as a cell phone) and if several phones are switched on during the landing phase, this may pose a very serious threat. I don't know, maybe an avionics engineer could shed some light on this. Is there any kind of research being done on this? I'm sure there is.
The question is, how seriously are airlines taking this? I'm sure it is difficult for airlines to implement rules since they don't want to impose strict codes on their patrons. But a certified jamming device is definitely worth it, if being worked on, when you have smart ass passengers who believe it is their domain to do whatever suits them since they've shelled out a fare for their carriage.