There's a statement from theymos on this if I can remember correctly, I can't find it right now.
Yep, this was his post.
However, if you are using the forum as a publishing platform to host something really substantial and useful, selling ads in that substantial work is allowed. To be eligible for this, your post must be in a topic that you started, and your post must be substantial and long enough to make the ad seem entirely insignificant. If in doubt, ask me.