This isn't a star sighting, but I did recently get contacted by a guy I used to go to hs with who, at the time, was very quiet, very nice, and who is now a Hollywood trotter.
He's now a model with pictures all over facebook and myspace that include parties he's been to with such go-ers as Paris Hilton and Lance Bass. One photo shows Hilton kinda messed up and (I think) standing on a table. Certainly not a shot you'd see in US.
So, I've seen a quasi-famous/famous by association person, but not on a plane. Plus, you know, we actually had regular conversations like regular people (albeit in high school...
).