They absolutely will not approve sites that force a user to register in order to read any posts or threads on it. They also will not approve sites that are warez sites, adult sites or adult related. So if you are running an adult site, a warez site or you HAVE to register to read the site you will not get adsense at all.
interesting....so what should I do then? hide the threads that require you to be registered??
That may not work at all. They more and likely require to see everything. Your best bet would be to use adbrite or put an affiliate in place and forget about adsense, unless you comply to their terms you will not get an account.
You can have private forums, just hide them so the public doesn't see them at all- no one will know they exist but the people with access to them. But really a couple of private forums shouldn't be a problem.