I always say hi, genuine smile... from there, it depends on the day. If I've been talking in client meetings all day, I'm not really up for being 'on' again. An Ipod does work wonders to send a subtle clue - with that said, I also tend to try and be aware of their reason for conversation - maybe they are a really nervous flyer, and talking a bit helps them to relax - maybe its been a bad day for them and they just need to get something off their chest - I really don't mind being a sounding board then as its most likely only a couple of hours of my time, and I do get plenty of alone time in hotel rooms, rental cars, etc to more than make up for the 'lost' time on a plane. I think we've all been in a position where its nice to just have someone to listen to us.