Where do the pico cells connect to? There are other complications too, such as aircraft speed. So you would need to put the pico cells in place for 700MHz (White space), 900 MHz (GSM) 1800 MHz (GSM), 2.3 - 2.4 GHz (G5 downloading), 2.6 - 2.7 (G4) and possibly something at 3GHz or so. Then you need make sure that whatever you relay to, the distances don't exceed the maxima that the systems can handle (GSM has a maximum of 40 to 50km becasue after that the timing runs out)
So now you need a ground station system....... plus a lot more weight in aircraft avionics. Of course, when you charge £1 a minute or whatever, it would pay for itself until consumers caught on - I gather that already, people are wondering about the cost to them of G4. I suspect that there will be some heartache when the bills come through - perhaps even for the corporate 'phone users.