Interesting topic. I see people getting peeved at people not spilling, but I think that there still is a code of ethics involved when it comes to celebrities. I'm not personally what I would call "friends" with any known people, though there are some that know me on a first name basis.
There's something cool and respectful about NOT talking about it, or risk being looked at as a name-dropping a-hole. It means different things to different people.
I once knew someone who got to know a Hollywood actor. He came to a movie screening, and it was weird to hear him call my friend by name.
If someone won't mention something, it could be simply a respect thing, not trying to keep anything from anyone, or any other "look who I hung out with" BS.
Dan/NS