Generally, there's a rule of thumb going around, saying "Don't post anything in public on the Internet that you wouldn't want to have your mother read on the front page of the New York Times."
While a reasonable rule of thumb, it's not a cut-and-dried answer. Take, hypothetically, this very post I'm typing right now, which I'm sending to "Public". Suppose it shows up on the front page of the NYT and my mother reads it. I don't have a problem with her realizing that I'm writing this, nor about the content of this. But if such a scenario were actually to happen, my mother would ask a bunch of follow-up questions, such as:
"Why did the New York Times decide to print your post? Are you considered very important for some reason? Did you get paid for writing it? Should you be? Did you give them permission? How come none of your other friends got front-page articles in the New York Times? Why are you portraying me like a nagger in your post? You are really embarrassing me by talking about me in the New York Times."
Now those questions, I would find very annoying. (Okay, the last one isn't a question.) So to answer the original rule-of-thumb, I'd have to say, yeah, I don't want my mother reading this post on the front page of the New York Times. But that seems like a very poor reason to stop the post from being public. What matters is profile and context -- my post may be very innocuous, but by placing it on the front page of the NYT, it introduces connotations that aren't in the post itself, namely that somehow I'm more important than all the other ranters on the Internet or that what I have to say is more important somehow.
If, say, the front page of the NYT was in the general habit of printing a random blog entry every day on their front page, then things are different. I can just say "Hey Mom, it was just random, I'm not anyone special." She'll still have annoying questions but I can deal with those.
So, what I'm trying to illustrate with this is that, when making a decision on whether to make something public, context and profile matter. Privacy is not a binary decision. There is plenty of technically publicly-available information that is de facto private.
But everyone knows that. What is more subtle is that this distinction works at smaller scales. My e-mail address is public -- anyone who is dedicated enough can easily find it and send me an e-mail. I don't mind that someone who honestly wants to send me a message can. But I do mind if my e-mail shows up on top of everybody's radar, saying "hey I think you should e-mail Wei-Hwa". Or, for another example, if you dig hard enough you can make some pretty accurate estimates about my wealth. I don't care enough to hide it, but I don't want it broadcasted either to make me an easy target for spammers or worse.
I'm not sure how much people who are worried about privacy issues are thinking about these shades of gray. From all their discourse I sure get the feeling that everyone prefers to see privacy issues in black-and-white.