Just typed 'Jay' in the 'guest name' field It allowed me to post under that name, but not register the name.
No You will notice that it doesn't display the type of user for Jay (e.g. "New User" etc) or the number of posts or a link to his profile. Once people get the hang of it you can see who is a registered user and when someon is a guest.
ah! gotcha! So at least we can tell what is from a registered user - therefore somebody posting as me but not me will be noticeable! That's good then