Maybe not so much of a celebrity now, but Carol Smillie. My sister was unfortunate to sit next to her when flying up from London only to be told "don't even thing about engaging in any conversation with me..."
Funny she was like that, as I've met her a few times in real life and Carol's always been charming. Plus on Twitter she replies and chats away quite happily to people.
Again maybe one of those days when a celeb is tired and just doesn't wanna know. Gee I'm like that on flights a lot of the time, especially if it's a work trip