Four important points to understand about cell phones on planes:

1) Of course a cell phone about to ring next to cockpit equipment will cause interference. If one is next to your speakers at home, it does the same thing. This effect does not effect equipment more than 3 meters away however...

2) The Skyphone people (GTE, ATT, etc.) do not want passengers to use them as they would destroy revenue. The technology used in the US is nothing more than cell technology, just multiplied by many times and using different frequencies (as opposed to overseas flights which use a sat based system). Contrary to popular beliefs, these phones only use different frequencies and are unshielded as many believe.

3) Airlines, don't want cell phones as they cause great annoyance to passengers who don't use them and would incite air rage if put into use.

4) Depending on its position over Europe, as an example, a cell phone at 30,000 feet above Europe can be available to as many as 2,500 cell stations at one time (normally your cell phone is available to 1-10 stations at a time and generally from one operator in one country), therefore a a user on a plane cruising at 550MPH has the capability to actually blow out large parts of cell networks in several countries just by having his phone on! The same principal applies just about everywhere. In recent years the software has improved and has the tracking and this risk has become more remote, it is still however a small possibility.

This is the biggest reason for not using them on planes.