I have added code to limit comments to being seen only by members

in
posts.main.php, page.main.php and single.main.php

Example: in single.main (version $Id: single.main.php,v 1.7 2007/11/24 21:31:22 fplanque Exp $)

before
skin_include( '_item_feedback.inc.php', array

if(isset($current_User) and ($current_User->level)>1){
skin_include( '_item_feedback.inc.php', array

and put a banner informing users of this when not logged in
After <div id="content" class="column">

<?php
if(!isset($current_User) )
{
echo ('<div class="page_action">Only logged in users can see comments to posts')</div>');
}
?>