I've never seen a guest post something, but I was looking at the online user list, and I saw this. I don't know if they can or not but it looked a bit weird to me.
edit: There happens to be no user called "Guest". Its probably a glitch and it was someone else who posted but it said "Guest" instead of their username
Honestly guys - all you had to do is log out and then try posting in one of the topics. You get a board message that explains you don't have permission to post and asks you to log in.
I hope you don't mind, but I'll just close this one so we don't wander around in circles with similar answers or wild speculations