You only have to be standing in the flight deck on turnround to realise the effect......a second or so before a mobile starts ringing you get interference and crackling on the instruments / speakers.
747Focal - There is a big difference between the "thousands of signals" passing through the aircraft as you come in to land, compared with a cellphone in the forward lav....as indicated by another poster.
I've heard cellphones beeping/ringing as an aircraft is on final approach (and an embarrassed passenger who couldn't undo seatbelt to switch off said phone!). Could this have an impact if you were on a Cat IIIB approach? Probably not, but I'd rather not take the chance!