There are actually two reasons they make you turn off your cell phones.

Yes, there's some chance that your BB/Treo/un-named-gadget will interfere with aircraft electronics, particularly the proximity detectors and stuff that makes airplanes fly on their own. That's what they tell you, but it's only the secondary reason.

The primary reason is what your cell phone will do to carrier's cell network while at 35,000 feet and traveling at... however fast that is. That's the reason FCC is more worried about cell phones on airplanes than FAA. See, the real beauty of cell phone networks is that (a) it knows where you are so it can route the digital traffic to your handset; and (b) it can switch from one tower to another, as you drive or walk down the street.

When you're at 35,000 feet, your phone will see a lot of towers, and will be tempted to switch between them very rapidly. A few cell phones accidentally left on by... well, all of us have done it, I am sure... isn't going to make a huge impact. But, there is some very justified fear that, if everyone will leave their phones on in the airplanes, it will jam up the network on the ground.

Not to mention that most of today's phones aren't designed to switch between towers that quickly, which is the reason you would have trouble making calls in the air.