Could you just have a board where guests can post?
Yeah, you always could. That's partly why I was wondering, a forum board could be turned into a guestbook easily, I guess.
But, having an actual guestbook would look more, I don't know, professional or robust or something. As a module, for TP, it could only add to the functionality and greatness of TP, I suppose.
It still seems that many, including myself, would prefer to have both, as opposed to just converting a board in a forum into something with similar functionality. It would still be the forum and not a guestbook, though.