The very slim possibility of interference is one of the main reasons they disallow them during take off and landing, since that is the most critical part of the flight, the other reason is passenger distraction, they want people paying attention.

http://www.wired.com/techbiz/media/news/2001/02/41273

http://en.wikipedia.org/wiki/Mobile_phones_on_aircraft#Elect...

The reason it is unlikely is because the systems are shielded but you could for see a situation where shoddy maintenance leads to the degradation or removal of what ever is shielding the instrument.