Publisher is not a web design tool, like FrontPage. It only supports simple
static web pages. Nothing dynamic
(http://www.publishermvps.com/Default.aspx?tabid=30) - it only supports
simple forms by using FrontPage technology on the server. A guestbook is a
non static entity that requires a back and forth communication between the
client (front-end) and the server (back-end).  As this functionality is out
of scope for Publisher it is not under consideration. You can use third party
providers though, a common one is http://www.bravenet.com , with your
Publisher web pages.