What a coincidence this thread has been bumped back up. Was in the USA last week and caught an IAD-BNA flight at 22:00 on the day I arrived into the US. So was completely shattered, it being 03:00AM as far as my body clock was concerned.

Earlier at LHR I had been given my boarding pass for this connecting flight and had specifically requested an exit Window seat for the extra room. So I board at IAD to see my seat occupied by a girl who's husband / partner was in the adjacent seat.

"Excuse me, you are in my seat" said I very politely.
"But we want to sit together. You can have my seat" and she pointed to the non-exit window seat behind. In the adjacent seat to that one was one huge chap, his body mass spilling over into the window seat space.
"Well, I requested this seat as it is exit row so I would prefer to sit there", I said.
"Lets ask the flight attendant whether he can be moved" she astonishingly said to her partner.
"Look, the flight attendant does not allocate the seats, this is done at check-in. If you wanted adjacent seats you could have got them by arriving earlier. Just like I did" I said, now raising my voice.

There then followed lots of grumbling and glaring by the girl and her partner who made a big thing of getting her books and mags out of the seat pocket and taking as long as possible. I settled into my seat and just went to sleep, glad of the extra legroom to stretch out.

It's all about how you ask I think. I would have no objection normally after I had sat down to someone asking me to consider moving to an equivalent seat, ie definitely not a window / aisle to a middle. But when you board and someone is already in your seat and then just expects you to accomodate their preferences I refuse. Particularly when she expected the flight attendant to intervene and demand that I move simply to accomodate her wants. At that point she had no chance.