I've updated the showthreaded.tmpl file so that it has the same reply link that is on the showflat pages. If a user is not logged in, and the post they want to reply to is in a forum that does not allow unregistered users to post, then the reply hyperlink takes them to the login page where they can either log in if they have an account or use the register hyperlink to become a member of your site. The same reply link was already in the showflat.tmpl file, I just forgot to add it to the threaded view
If you are already using the templates then just redownload the zip file attached to the first post in this thread, extract the showthreaded.tmpl file, and upload it to your templates/default directory.
