I won't mention the ones outside of airplanes, as I grew up in Palm Springs and have seen/met/know far too many celebrities.
However, I was sitting in the last row of FC on a CO SEA-EWR flight. The other three seats there were members of Kool & the Gang (remember them?), who were on their way to Paris. I tried not to hound Dylan Thomas (sax player), who sat next to me.
So we ended up chatting about "normal" seatmate stuff, and he cringed a little when he told me what he did. We talked about music and stuff (I'm not accomplished as a musician, but I write, play and even occasionally perform), and what it's like having days on the road with brief performances to break the monotony. All in all, a pleasant experience for both of us (at least, I THINK he had a pleasant experience...he seemed genuinely pleased).