Actually, some Boeing engineers ran informal tests to see what if any interference is caused by cell phones. They could not find any problems.
The real problem comes from the FCC and cell phone companies. Cell phone usage on airplanes does cause cause problems for users on the ground. The problem is that when you are 6-7 miles high, you can "see" alot of towers. This causes blocked calls for the terrestrial users which causes headaches for the providers. Incidently, I read a few days ago that the FAA is going to reevaulate the cell phone concern. I still don't think they will lift the ban, but they may inject a little common sense in it.