In-Flight Phone Use

The real reason why you have to turn off your cellphone when flying:

The Federal Communications Commission in Washington currently prohibits in-flight cellphone use on planes, partly because of some unresolved questions about the potential for interference with aircraft navigation equipment, but mostly because of phone industry concerns that airborne cell signals radiate widely, randomly contacting different ground stations. That would create interference between systems and cause logistical problems for things like billing.

From the NYT. Resistance to allowing cell phone use is still strong in the U.S., though it's becoming common overseas.