Since late 2002, I have used an email signature hack to explicitly tag the privacy of a thread:
this email is: [ ] blogable [ x ] ask first [ ] private
It works because [ x ] ask first is the default. I've only had two cases where someone didn't respect the privacy tagging of the email and blogged something.
Now I've made it easier to change to [ x ] blogable (sometimes bloggable) or [ x ] private through three different email signatures. For Mac users, click on the images for a how to in Mail.app preferences.