Nick's response to the question is certainly how I understood the position re: the aircraft. However, I believe that the network operators do not like phones connected to their network being used at altitude because one phone can "occupy" more then one of their cells.
I am a bit worried by the current position though. Mobiles are now so common, and it is inevitable that some will be left on aboard aircraft (intentionally or otherwise). If Nick is correct that any phone being left on has a small risk of affecting aircraft systems, all those cases must eventually lead to a phone affecting flight critical systems with a disastrous result. Shouldn't the critical systems be designed to be resistant to such effects??