Do your friends, family and work colleagues know that you play role-playing games?
Yes. And I know they like to take photos, or read up on submarines, or create avant garde movies, or play in a trash metal band in their spare time.
Gaming is a way to pass time, on par with any other hobby.
but what about casual acquaintances or family members you only see a couple of times per year?
Distant relatives knows sort of that I have interests in gaming. They wouldn't know D&D from WoW even if the game suddenly acquired a real persona and stabbed them in the back, but they know I'm a gamer.
Come to think of it, I think my distant family knows a lot more about what I do for fun than I know about their hobbies. Do you know what your uncle's hobby is? I haven't got the faintest clue.
Casual aquaintances? If the subject comes up, I tell them. Most often the casual nature of the conversations excludes topics relating to hobbies, and are more concerned with the weather and upcoming holidays. And maybe sports results.
More to the point in my case, what about colleagues from work?
They can't really miss when I come back from the FLGS with a big honking boxed game.
Is it wrong to hide your hobby if you think people will look down on you for it?
I don't think there's anything wrong with keeping your hobby private, for whatever reason. Most people don't care, won't remember what it is you do even if you tell them.
Do you know your colleagues hobbies? Does it bother you if you know, or does it bother you if you don't know?
Don't sweat it, is my advice. Keep it private if you want to. No big deal.
I get far more grief for listening to Genesis and using a Macintosh computer than I get for playing D&D.
EDIT: The really strange things is that I like Genesis with and without Peter.
/M