The only relatives I have as friends on Facebook are my sister and cousins, which doesn't bother me at all. If my parents joined the network, I don't think I'd let them add me. I don't really have anything on my profile that I feel I'd need to hide, but they're my parents, not my friends, and I'd prefer to keep my family life and social life separate -- for the most part, anyway. I'm on pretty good terms with my parents, but I don't think they need to keep up to date on everything I decide to post on Facebook -- as others have said, that might be a bit too clingy.
I don't think I have much to worry about, though -- my mom only figured out how to use e-mail a couple years ago, and she still hasn't completely mastered it yet. I doubt she's planning on joining Facebook anytime soon.