Mig, I'd be very surprised if the cost was similar to international roaming. It may well be cheaper than the $10/min for the satellite phone, but my guess would be not by much (the airline is effectively paying for a satellite call between the plane and their mobile service provider).
Does anyone know if these systems are going to be quad band? My (limited) understanding is that they (the European ones, anyway) will be 'GSM' only. I can picture a scene where half the pax wanting to use phones are futilely trying to get a signal from a base station on the wrong frequency (and in mid-Atlantic, the distance to the nearest compatible base station may be 10 km vertically and 2,000 km horizontally!)