I want something like if someone is unregistered they're shown a message like "Hi, you may need to register before viewing the contents of this forum".
Does that exist?
He probably want to "hide" the content of the forums before login or register not just putting a notice of login or register.
You might do this manually, by removing the permissions of "view forums" and "view threads within forums" in guests usergroup,. but that show "No permissions error" message.
Its better to do with a plugin.
Drop me a PM if you want me to make this for you or try to post this as a request so I may remember it:
http://yaldaram.com/forumdisplay.php?fid=3