OK, as a former mobile phone engineer for Cellnet, here is the deal.
There is liitle evidence to suggest that a phone will intefere with aircraft systems, as far as there is little to suggest that making a call in a petrol station will result in your anhialation. The probability is small, but would you really want to find out????
The "real" reason has already been given, albeit in the usual sort of sensational language that one reserves for jounalistts. Contrary to popular beleif, when you make a call, it does not go blasting off to the nearest satellite, neither does it scramble your brain.
It works like this. When you make a call, the phone samples what you say upto 7 times per second, and transmits this as a data stream to the network at a rate of 9600 bps. (At the other end, the computer fills in the gaps, and it sounds like you again) To enable this transmission to succeed at this rate, the phone will either speed up or slow down the transmission speed, so that when the signal gets to the network, it is travelling at 9600bps.
The speeding up or slowing down is dependent on where you are in realtion to a cellsite. ( A network receiving station) Due to this, the phone has a maximum range of about 10 kilometers. Beyond this, the signal reaches the cellsite too slowly, and the person at the other end thinks they are talking to a dalek. Too far away, and the computer cannot cope anymore, and drops the call altogether.
Now then, the cellsites are designed, as are the phones, for a land based existence. However, as I am sure you can all remeber from your R/T studies, radio waves move in all directions, and this inculdes UP. Because the ground is littered with cellsites, and don't forget that the "line of sight" rules apply in the main, even with a mobile phone, there is plenty of coverage at ground level, and the system can cope with a phone that can see 3 or 4 sites. Go up a few thousand feet, and the phone can now see hundreds of sites, and tries to talk to them all at once. Result, as has already been said, the network will go on strike tout suite.
Hope this clears it up.