The few occasions that someone does ask me 'Can I ask you a personal question?' I'm always curious as to what it is they're wondering.
So usually I'll say go ahead, but I may not answer depending on the question.
And strangely enough, just saying that in response deters so many of them for some reason.
Depending on who it is, I most likely won't answer. I'm a little more open online though, since people there can't hurt me as much as in real life.
Either way though, part of me is happy when someone asks personal questions because it makes me feel a little less invisible.