I have read that having adsense on the forum helps in getting the forum listed quicker in the search engines as the spiders crawl the page more often to read the content and match the ads thus listing even the deeper pages of the forum.Other than that, I feel advertising shouldn't be allowed in the main sections.Otherwise, it's like an open invitation to spammers.Allowing members to show their signatures is fine, but for advertising purposes there should be a separate section in the forum.