I'm a few days late posting this, but the newest version of dhGuestbook has been released. In this version a delete comment option has been added for the administrator. Upgrading is simple (replace 4 lines and add values for 2 lines) and installing it for the first time is really simple!
So if you're looking for a free PHP Guestbook (that uses a MySQL backend) then go and download it now. It's all free and if you need any help with it, then use the contact form and I'll help you. Many people around the internet are using it too, which is brilliant!