Nowadays, we always have to be careful about what we say, because we never know what might come back and haunt us. All you need is someone with a bit too much time on their hands and a slightly warped sense of righteousness, and you might find yourself defending your ass in court.
The EFF just blogged about the case of some guy suing Craigslist for defamation because of some other person's post. Craigslist removed the posts, but not quick enough, apparently.
The lawsuit rests on the fact that Craigslist supposedly told the guy that they'll "take care of it", which, it would seem, implies that Craigslist promised the guy that nothing bad about him will ever get posted ever ever until the end of time.
We'll have to see what the trial judge says, but in the meantime, be careful what you tell other people.