I recently flew in 64A from LHR-SFO and I was asked at check-in and again on the plane if I was willing to move to allow a couple with a baby to sit there. It was made clear that I was under no obligation to move and I simply said that I would move if they had a seat as good as 64A to move me into. They smiled and said they'd leave me where I was.

The couple with the baby sat on the UD and I did get the feeling that they were glaring at me:-)