Oh, absolutely. I agree. Definitely useful for managing the liars who change their post content after-the-fact, and also for the ones that delete all their posts on the way out!!
It'll be more than welcome if/when it makes it to XF.
Oh, sorry. In my post above I was saying about a note that the message has been edited (this post was edited by XXX on XXX).
Post edit history important feature too, but it's probably not an easy task.
Yes GeeksChat is right, IPB does not have this. It will stick an optional "edited by" note after a post but unlike vB it is not clickable and you don't have any access to a log which shows what was edited. Even the IPB admin/mod logs just show the action was done, but not what was edited. vB really is the clear leader in this area, giving access to versions of the edited posts including the original. With XF lacking this you really do need to trust your moderators!