Hmm, I can look at this two ways.
A member's only area that is publicly labeled as such could work two ways:
1) Could peak a new user's curiosity and drive registration OR
2) Could be a turnoff/unfriendly for people that regularly visit the site but do not prefer to log in
Typically, when you're trying to attract new readers/participants/customers, the fewer hoops they have to pass through the better. The appeal of the forum is that it is a friendly, caring and safe place for people to improve their style and self esteem. The wonderful thing about it is that it is open to ALL, no matter whether you are just starting to take care of yourself for the first time or you're a style guru.
I wouldn't want a members only area to be looked at like a place for frequent posters or style gurus only!
I like the idea of posts that are designated as "private" by the author being viewable as a pleasant surprise when you log in, just like how we can see the conversation status bubbles (green, gray or white). You don't really miss them when you are not logged in, but nice to see once you're there!
Personally, I don't think that people should post really sensitive information online if they have privacy concerns. I figure if you're putting on a website, you're okay with the info being out in the world. And I hate it when I have to log in to a new site to access content.
I would not make the privacy flag a default. I would make it a checkbox that has to be rechecked indicating the request for privacy each time a user posts.
PS - BTW, I love thinking about this stuff because I'm in marketing and it's my job so I hope I'm not offending anyone. I'm going to continue logging in all the time regardless