I received one of these emails tonight. My site covers a wide variety of topics and has user forums. As it happens the URL they gave me was a visitor post that hadn't been caught by the filters or picked up manually. The page in question was perhaps borderline and did attract some stupid comments but the question is, how can you be sure that there isn't something else lurking out there that will get you into trouble one day. With thousands of comments posted every week it's difficult to catch them all and the 'mature content' thing is quite a grey area. For example, is talking about alcohol free beer a mature subject?
I've been in the AdSense business since about 2005 and always tried to make sure things are clean and this time I got burnt. Interestingly enough, ad serving hasn't been disabled site wide yet - perhaps that takes time to kick in. Anyway I've completely removed the page and all related comments and can't think of anything else that might be a problem. Here's hoping.... I'd hate to have to get a 9-5 again.