Cell phones do not cause a problem with avoinics but the frequency is only approved for ground to ground communications by our friends at the FCC. This is the reason cell phones are not allowed to be used on planes, that and it really will mess up the cell towers. At least this is my understanding of the issue, and as my wife tells me I am often wrong.
The cell phone ban drives me nuts, so I've looked into it, and as near as I can tell, the history of it is:
1) Cell phones come out. People start taking them everywhere. Some people use them on planes.
2) Cell phone companies find that the cell towers can't handle users roaming between cells as quickly as planes fly.
3) Cell phone companies go to the FAA and say "Please ban cell phones on planes"
4) FAA says "Sure, why not? Doesn't hurt us." They ban cell phones on planes.
5) A few years later, airlines see this market for making calls from planes, so they put those expensive little seatback phones in there.
6) Cell phone companies finally get with the program, and design their cell towers so they can more or less handle people roaming as quickly as they like.
7) Cell phone companies go to the FAA and say "Please lift the cell phone ban on planes"
8) The FAA doesn't actually have anything against this, but the airlines go whine to the FAA and ask them not to lift the ban.
9) The FAA compromises, and says that pilots can ban whatever electronics they want on planes, whenever they want. [*]
10) Airlines then tell pilots "You'll ban cell phones, or we'll fire you"
[*] This isn't, AFAIK, actually public. I've seen a copy of an FAA document describing this policy, it wasn't marked as being intended for public release. I'm not sure how to find it any more, though.
Perhaps I'm wrong. There aren't any guarantees in life, of course. But I don't think I'm wrong, and the story works. As always, you've got to judge for yourself.
(But if you doubt me, consider this: The cell towers emit vastly more RF energy than cell phones, and some airports have cell towers awfully close to them. And you can use a cell phone right by the gate door while a plane is taking off. (Supposedly its most sensitive time.) And would the FAA really let a plane fly if its avionics could be disrupted by a device that contains at most a single amp hour of power? The first jammer someone built would bring every plane out of the sky.)
They are illegal in the U.S., not because of any "right" to engage in cell phone conversation, but because cell phone calls are considered radio transmissions, and interfering with radio transmissions is illegal.
Only active jamming is illegal. I'm pretty sure you can build a Faraday cage into the walls of your building if you feel like it.
They ban the use of these items upon take-off and landing where things are most likely to go awry. They do not want notebook computers flying around the plane, nor do they want people to be blasting musuc through headphones or talking on cell phones while you are supposed to be "following crew member instructions." It is about control.
Don't ask how I know. 
Achnod with some C4 and a cell? Piece of cake.
