Hi Jon,
I quite agree with the other responders. Allowing JS in forum posts would create a bigger hole in security than anything Bill Gates could ever build into Windows.
Consider this scenario: A less-than-scrupulous poster could embed a snippet of JS in their post that surreptitiously (without being visibly noticed) emails spam to 10 million email addresses using your server's sendmail function (and IP address) to do it.
Guess who's going to get shut down as a result of all of the spam complaints? (Hint: It won't be the poster...)
I find it somewhat intriguing that you wish to permit including Adsense within the actual content of a forum, since Adsense ads tend to be oriented and adapted toward the actual subject matter of the page they're placed on (according to Google's secret family recipe). I assume you're wanting to help your members accrue Adsense income through their posts in your forum, which is actually quite admirable in principle.
However the risks far outweigh the benefit, in my opinion.