Not really on topic, I just thought this was funny at the time: I heard an interview with Carl Lewis, and he got asked some version of the OP question. He said that he is constantly approached by people asking him to race them. Right now. Down the airport hallway or wherever.
Care to name some examples of both?
I once met Stephen Sondheim (my then-bf knew him), and I tried real hard not to gush and fawn over him.
Me: “I’m not going to say what a huge fan I am of your work.”
SS: “Everyone says they’re not going to say that.”
Sure.
Dicks: Jennifer Lopez, Transiberian Orchestra, the Police, Van Halen(though I did get to drive Valerie Bertinelli across town from her hotel to the venue:)), Jonas Brothers.
Cool: Faith Hill(talked to her at lunch, didn’t even recognize her; thought she was part of the crew), Meat Loaf, Ben Folds(played ping-pong with him for an hour), Widespread Panic, Weird Al Yankovic(let me try on the fat suit:D).
The Jonas Brothers, that one kind of surprises me.
Maybe they’re not normally like that, but on that particular day at least, they were very aloof.
Of course, everyone has good days and bad days. Most people’s bad days aren’t discussed by strangers on the Internet.
(No, I’m not a Jonas Brothers fan. Just bored.)
Only two surprises for me, really: Trans-Siberian Orchestra and Faith Hill (I’ve always labored under the assumption that she’s a giant asshole; good to hear that I was wrong and she’s not). And the only member of the Police that I could see not being an unconscionable penis is Andy Summers. And I’m a big fan of the group (especially Stewart Copeland, a.k.a. my favorite drummer of all time).
But the really important question is, was Ben Folds good at ping-pong? I’m betting he was.
Judge Reinhold, the actor who did the scene in Fast Times at Ridgemont High where he was caught pleasuring himself, has said that there were a few times in a crowded airport or some such where someone would recognize him, point, and shout out to a friend, “There’s the guy who jerked off!”
Just remembered, I have my own (slight) take on the OP’s question. Back in 1991, I was in the midst of a full on Metallica mania, and scored fifth row tickets to see them in Indianapolis at the start of their Wherever I May Roam tour. During one of the songs, James Hetfield (my favorite member of the group by a long shot) was standing right over me, and I looked up at him and had a pretty intense, maniacal smile on my face as I pumped my fist and screamed along to the song. His reaction to my slavering devotion to his music? A “WTF” grimace and a slight shaking of the head, after which he fixed his gaze elsewhere. At the time I thought it was pretty dick-ish, but looking back on it, I’d have probably reacted in much the same manner. Or not. Who knows?
(Plus, he was probably pretty insanely plastered at the time.)
Tell him you liked him as Speed Walker.
They brought their own table and everything, and I suck at ping-pong, but I managed to barely win a couple of games. So I guess the answer is no.
As self evident as this may seem, I can’t resist the impulse to nitpick.
There are a few occasions where you should bother anybody when they’re in the bathroom, celebrity or not.
One that springs to mind is “Sorry if I’m bothering you, but I just wanted to make sure you realized that a large part of the building you’re in is on fire.”
Even then, should one happen to encounter Rock Master Scott in the bathroom in this situation, one probably should not approach him with, “The roof! The roof! The roof is on fire!”
Had a boss that would follow me into the men’s room, knowing he’d have a captive audience as he pitched his latest Great Idea To Pull In More Business…
… Hey, I guess I was a celebrity!
I always think that if I run into someone famous, I just want to say “Thank you for the awesome-ness you’ve brought into my life.”
Then again, based on my one encounter (Davy Jones!) I will just get sweaty and tounge-tied and sound like a moron. 
Don’t know what I’d to if I ran into a celeb out of the blue; however, if I know I’ll be meeting one, I try to come up with something unique to say or ask. I asked Tony LaRussa which St. Louis Restaurant had the best vegetarian food and I asked David Sedaris if he was still good at guessing ages.
I rarely meet celebrities, and when I do it’s usually authors, which leave me tongue-tied! I usually just tell them how much I admire their talent and how much I enjoy their work. Authors are usually pretty happy to talk about their work, but the really famous ones do go all “I’ve heard this a thousand times :rolleyes:” on ya.
The biggest celebrity I had any contact with was Tommy Smothers, and I got to spend two whole days with him! It was a work situation; we were working together so ‘gushing’ in any way would have been inappropriate … but I always like to mention him because he was so nice. He showed me his yo-yos. 
And in other news, I just read that Alanis has married her boyfriend, rapper Mario “MC Souleye” Treadway (in spite of his name).