Here's a link to an interesting article I found on the web... It's a look at two mysterious cases of possession by demons and possible explanations for what happened. Even in the most extreme cases of reported demonic possession, isn't there the possibility of a more rational explanation?
http://beyondtheblog.wordpress.com/2007/09/23/a-demon-calls/
Good article. I found the comments to the article particularly interesting. Some very deep conversations.