Richard Chappell has a perfectly reasonable idea for apportioning airline luggage allowances: calculate a total per-passenger weight allotment, and charge those who exceed it.
This seems to me to be significantly more fair than both charging by the bag for extra luggage and/or charging the overweight for two seats, which seems to be done completely haphazardly. By weighing every traveler with their bags, a true weight can be established and charged for accordingly without anyone feeling that they have been singled out.
Of course, in the case of the obese passenger, this does nothing to address the real annoyance these passengers cause to their fellow travelers as they ooze out of the confines of their seat and into the seats on either side of them, but it is a start in the right direction.