It is not only the safety aspect from the aeroplane point of view. There is the legal stance from the telecommunications act (or whatever) in that it is illegal to use the mobile phone on the aircraft.
However, what I would like to really point out its that if you do insist (like 747FOCAL) on using the phone while in the air, don't be too surprised when your phone stops working. We all know that the VHF radio on the aircraft is line of sight, and the higher you get the more sight you get. The same for the mobile phone, except that it isn't designed to communicate with multiple base stations (which will occur), and the network may choose to shut down the channel, or the phone may be instructed to do opposing things by different base stations.
Ranting ends.