[@Sanctus Spooki]

Guests could be users that are offline or on invisible mode, but it can also be google, yahoo, or other search engine bots that help direct searches to the page. It's not anything to be too concerned about. :)

Edit: Also, the pages can keep a memory of people watching. I've noticed before that I've looked at a thread, logged in and it's myself and a guest. One is your non-logged in session, and one is the logged in session with the generated cookie.