This tutorial describes a PHP-Nuke Function to restrict access by page as public or private. The function pageStatus(); is called on the top (or inside your root level header, not the themes header) of all pages. This will allow you to deifne in your admin panel specifc pages and urls that you want to have a particular viewing status. It explains a few ways to go about defining which pages to have the "public" or "private" status. Also the code for the function is available.