I have to say that if I ever was on a flight, and the pax next to me wanted to raise the armrest as they were too big for the seat, I would immediately seek to move seats, or get off the plane. There are two reasons for this; Firstly I want to be able to get out of my row of seats in an emergency, and secondly, I paid for a whole seat, not a part of a seat.
I believe Southwest has a useful policy regarding this whereby larger people can purchase two adjacent seats. This is a good short term solution to a difficult problem, as it avoids any bad feeling with other pax, and means the person of larger girth does not find themselves in a 'no-win' situation with other pax. Maybe the likes of Ryanair or Ezy and other airlines need to wise up to this.
Long term, the airlines do need to address this issue, as people are just getting bigger. Fact.
Just out of curiosity (as I'll be flying in a middle seat on Jet blue shortly on a long flight), can someoen tell me, how much wider the A320 cabin is over say a 737 400 etc..