One big thing against forced placed ads is with php scripts. I don't know about others, but with a lot my scripts I use header() a lot. If anything is outputted before header is called, an error is triggered and the cookies/HTTP headers are not set/sent, which is a pain. (There are ways around this, with ob_start() and all, but that can be another problem.. ) I would think that the forced ads would be considered output. (I'm not 100% sure how the ads would be forced. If it would use auto_prepend_file in php.ini, then I'm not sure if that would be considered output.. Hmm)
Anyways, I voted for the "User Placed Ads" option. I still have the script that I started on the development account to check accounts for the ads. I can finish it easily, that is not problem. With that then, if ads are "put in" by users, account's can still be easily checked for ads. With the recent server cleanup that task becomes even easier.
Also like others have said if the option is picked for members to place their own ads, the problem comes up about members not knowing how to place their ads. I think this could be overcome with a "new" forum being created under the Support forum dedicated to helping members with *just* that. After a few weeks, I think there would be enough people's problems already taken care of in that forum, that other members could search the forum for older threads to help them out. (With placing their ads)
Well, those are my ideas.