Mythbusters did a test using a mobile phone and a panel-full of typical light-aircraft avionics and found that the mobile very definitely created a heck of a lot of interference. They then discovered that the wiring they used had none of the insulation that aircraft wiring has that stops external radio interference, and once they'd swapped all the wiring over to the correct type the interference from the mobile phone stopped.
Now all you have to do is convince me that 100% of the insulation on 100% of the aircraft is perfect and you'll convince me that mobiles don't occasionally cause interference with the avionics.