Having worked as an assistant editor for HarperCollins Publishers in both HarperPrism (SF/fantasy imprint) & HarperEntertainment (movie/tv/sports imprint) a few years ago, I have had more than my fair share of celebrity encounters:
Chated with William Shatner for 5 minutes while my boss finished up a conference call. I know almost nothing about Trek, and he seemed to be happy to talk about the days news with me.
Had lunch with Clive Barker twice.
Ordered lunch for Lara Parker (Angelique of Dark Shadows)
Talked with Richard Karn (Al from Home Improvement)
Harlan Ellison yelled at me for not being fast enough to answer the phone
Met "T-Box" from TLC
talked with lots of sf/fantasy authors on the phone, email, through the mail
Outside of a work situation, the encounters go down dramatically:
Meet both Dave Matthews and Carter Beauford of the Dave Matthews band (they live in Charlottesville, VA where I now live)
Sat at the bar while my father had a beer with Levon Helm of The Band
Was invited to an after show party by Jerry Cantrell of Alice in Chains (I didn't go, had a test the next day)
Sat at the table next to Danny Aiello at a restaurant in NYC