It really is an issue much more complex than that and sadly, becuase most of the people telling you to turn off your phone do not understand the multiple reasons why you shouldn't use a cellphone, they just "fall back" on the interference reason.

There was an extensive discussion about mid-June 2001 of both the technical problems and the FAA/FCC issues in the United forum, but it was before the UA/AA forum troubles and I can't find it.

1.Cell phone and the cellular system are designed for terrestrial (ground based) use and REUSE of frequencies in very close quarters. At altitude planes see many cell towers and the system and phone get confused, calls can barely be made, disconnect, and the system below is massively overloaded by just a few phones doing it.
2. It is the FCC that prohibits the use of cell phone while in the air. All radio regulation have a prohibition exemption for emergencies. Hijacked planes probably qualify as an emergency.

3. If you recall from reports, they don't work very well, Ms. Olsen (AA77) had to call her husband back becuase they would get disconnected so quickly. Bad-hand offs, the airborne cellphone is a "hidden tranmitter" to much of the ground, and contant cycling between cells.
4. At least one of the "cell phone" conversations from the plane were actually using airphones. The fellow who called his mother in California from UA 93.

Additionally it has been pointed out that the hijackers were not using much of the navigational systems on the plane, and were probably flying totally VFR.

The cellular system is not designed for use at altitude (try using them from a high bluff or mountain top near a population center, they do the same thing), but the communications and quality of maintaining a call. Not just the RFI (radio frequency interference) issue, which is overused in this case but still a factor. Sadlt societies stance on RFI is that the radiating source is at fault, hence the cell phone is at fault. (Somewhat due to poor testing and the electronics lobby getting Part 15 and Part 68 J-type interference laws & policies passed).

Airphones are for planes, if you need to call, call. I do agree it is outrageous pricing, but, sorry to say, any griping about cellphones not being available in a plane is really just griping about price.