The pound-and-a-half weight of a current iPad sheds the tremendous amount of weight that would normally be needed for a conventional in-flight entertainment system built into the headrest, which on a typical 375-seat 767 could see the iPads pay for themselves within years, if not sooner.
Fuels savings by reducing weight is significant when you include the two pilot bags (50lbs), Cockpit Manuals (25lbs), and Flight Attendant Manuals ( 4 x 25lbs) that the iPAD will eventually replace. 175 lbs is the weight of a person.