Full explination of this hack is located here:
http://webwenchginger.com/2008/03/15/nicedit-customizations-in-wordpress/

Place the following code into your header.php file, just above the </head> tag:

========= Start copy after this line =====>
<script src="http://js.nicedit.com/nicEdit.js" type="text/javascript"></script>
<script type="text/javascript">
bkLib.onDomLoaded(function() {
          new nicEditor({fullPanel : true}).panelInstance('comment');
		  });
		  </script>
========= End copy above this line =====>

Enjoy your new rich text comments!

WordPress hack courtesy of www.webwenchginger.com