Kit, I don't run a forum, but I'm curious. If you do admin approval, what happens when you're gone on vacation, or to a seminar, etc?
The forums I frequent most commonly have very active admin/owners, who are on the run and in high demand, i.e. Lynn Terry. If they waited for admin approval on every new user request, there would become a huge backlog of requests, unless they also have a backup to themselves.