I don't know about UA, but on BA, I've done it a few times. Twice I got away with it (I'm 6'1" so I use that as an excuse if questioned with a pleading look on my face) and the last time I tried the flight attendant said "Sir, do you realize how much this seat costs? Unless you're willing to pay for a full fare business class ticket, I suggest you move back to your seat. You may not sit here." I tried using my height as an excuse and he wasn't amused.

[This message has been edited by bhatnasx (edited 07-18-2003).]