I read this thread with interest. I have flown AC for many years, domestically and transborder, but not yet overseas. I do have lots of experience flying to HNL and LHR/LGW on AA and other airlines, and if its one thing I deeply resent in my many years of flying is seat sharing between F/C and Y cabins!

Lately, post 9-11, I have rarely seen much (or attempts) on USA carriers, both domestically and internationally. Flight attendants with AA tell me there is a zero tolerance policy for seat sharing between cabins nowadays. When I fly with a companion, either we both upgrade, or we both stay in the back.

Years ago, I sat near a family who continously played musical chairs and switched seats from the back, especially after the dinner was served, so that everyone could get a f/c sundae for dessert. They were loud, obnoxious and disturbing.

En route to SYD, a Y passenger was allowed to move up to C class (after the first dinner service), to sit in an empty seat next to a friend, to play cards, and only moved back to coach in time for landing -- about 9 hours later!

Over a year ago, I flew from LGW-ATL on Delta and a squatter from coach was allowed to move up to sit next to a friend in business/first mid flight, and received premium service en route to Atlanta for the remainder of the journey.

I could probably recollect many more instances of seat-sharing and upgrading by squatting, but no matter what the circumstance, I think its low class, tacky, and offensive to pax like myself who dole out extra mileage points and/or upgrade fees!