Yes, I am aware of the ad. I have allowed one of the advertising companies MW uses to do "pop-under" ads because their system will only serve each user ONE pop-under per day. That's much more acceptable than some of the other places that not only do "pop-overs", but deliver them non-stop to every user. Forget that.
Any ways, I figured since it's limited to one pop-under ad per day it wouldn't be a major nusances to users and cause everyone to run for the hills...
Also, yeah... I think the classifieds will work out well this way. By making it built into the forums it gives the classifieds all the features of the forums (attachments/in-line photos, etc) and I think will generate more overall activity/exposure...