If you have to have that kind of flexibility then of course this system doesn't work. But usually, for a message board app, you don't need that. It's a "write once and leave it" kind of application. And let's say you do delete a message, the fact that it's gone doesn't destroy the relationships of all the other messages...
Comment on Re: Re: Re: Message Board Threading with DBI/MySQL
Even if it has 'child' messages, in case you have some kind of reply system? :) When deleting a message, you should also make sure to delete child messages, since orphan messages will get inaccessible if you dont (which ofcourse take up space, slow down queries, etc)
Greetz
Beatnik
... Quidquid perl dictum sit, altum viditur.