Yup. I am now a mod on my band's forums, and managed to out one of our members who was back in his home state. He thought he'd be able to post anonymously.

It took all of about two minutes to copy his IP (from the anon posts), run an InterNIC on it, and pin down where his server was. Got state and city right off that, which in this case was enough. If I'd needed to, I could very quickly have pulled a lot more.