I've thought about this as well.
Agreed that the best way to do it is to make the plane more or less autonomous, and then send course changes and such over a cell modem.
This is the way the Global Hawk and other long range UAVs work, except with a satellite phone modem instead of a cell phone. Looked into satellite modems, but the cost of the air time for one long mission would be $1000s.
Anyway, the tech to do something like this is not hard, but my question is, what is the cell reception like at altitude? My guess is that the cell towers are all optimized for ground level reception and that you'd get all kinds of dropout when you got higher up? Anybody know?
The clever way to implement this would be to have the plane continuously log GPS waypoints as long as it was getting good data from your groundstation, and then if there was a dropout, the plane would circle back to the last known good point of reception .. . . .