Postings on an open, public forum are not exactly private are they?! In fact anything that's freely published on the internet should be, and generally is, completely spiderable and indexable. That's the whole point!
Besides which, this is a just a free little forum on some company's servers. I couldn't change the headers or add a robots.txt even if I wanted to!
So anything that goes on this forum is archived and publically indexed, so watch what you say!