I was told that they were banned for two reasons, one already discussed their effect on the aircraft, but also because they are not licensed for such use by the governments/service providers because they cause havoc with the cellular principle on the ground.
When you move from one cell to another, either on foot or in a car, you are "handed over" to the next cell so that when anyone rings you the system knows where you are and which cell should try and contact you.
If you are at 35,000' and doing 500mph then you are in range of lots of cells because of you altitude, and the cells cannot cope and the amount of switching cells causes havoc in the system. Perhaps that is no longer true, but that is what I was told in the early days of mobile phones.