Do the mobile carriers (or the airlines) charge a premium for in flight calls? I don't think our ground infrastructure would be up the task in large metro areas such as the Boston to DC corridor unless the AC had a concentrator that relayed the signals similar to wifi.
Like I said, United used to have cellular - so did USAir if memory serves. But I suspect they made some profit from the wireless provider (originally GTE for UAL). They didn't fall out of the sky either though having tested and forced messages to appear on the G IV, I'm still concerned about crew distraction trying to diagnose spurious and incorrect display messages during the more critical phases of flight.