You know the feeling, everyone who is adminning forums is being asked questions through PM which really ought to be asked in the forum...

You can solve this in 2 ways: you ask people not to PM you for the wrong things or, and this is what this feature sugggestion is about...:

you put in the ability to promote a private thread (pm) to a public thread in the forum of your choice. It should be rather easy to put this in, but as this can be pretty powerfull you should be able to limit it on a permission mask basis...

I dunno about this idea, but I actually have a use for switching blog entries to posts or posts to blog entries, which is similar to what you say, in a way. lol