Why, WHY, WHY!?
There is NO Guestbook! If you really want to sign a guestbook, go to ABSOLUTELY ANY OTHER INTERNET SITE YOU CARE TO NAME (except a few, obviously) and add your scrawl to the masses. Nobody reads them. The only person that cares is the person who owns the site, and if you want to let them know how much you like it, send them an email instead - they'll be far more grateful.