Any overweight passengers, that cannot easily fit into a single seat, will be charged the price of an additional seat.  United has begun to flag overweight passengers and will not allow them to board without first purchasing an extra seat.  Their definition of "overweight" is if you are unable to comfortably fasten your seat belt or sit in your seat with the arm rests down.  This will be the case, even if the overweight passenger can be re-seated somewhere else in the aircraft next to an empty seat.

Southwest airlines made a similar decision in their policies a few years ago, but they have also been known to allow overweight customers to move next to an empty seat, if one is available, without the extra charge.  American airlines has spoken out against United's recent decision and has said they would not charge an overweight person extra unless no other suitable option could be found.