EDIT: If we're talking about celebrities (or famous people) we've encountered outside of WDW, then I've seen Stephen Hawking in my hometown (as he lives there too), and I went to University with someone who has since become a TV weather presenter (not going to say who).
Stephen Hawking is cool! Would be funny if you were on the bus and said "Not another ECV getting on!" and it was Stephen Hawking.
Thought of two more I heard speak in college: Vincent Price and Graham Chapman (blonde guy from Monty Python). In fact, I saw all of Monty Python live on stage in 1977, that was very cool for a young Python fan!
Oops, another - and at least this is sort of "park" related though not WDW - we waited on line for the sky-ride thingy at the Bronx Zoo with Jennifer Connelly (Labyrinth, Pollock, A Beautiful Mind, etc.).