Index: wp-includes/feed-rss2-comments.php
===================================================================
--- wp-includes/feed-rss2-comments.php	(revision 7262)
+++ wp-includes/feed-rss2-comments.php	(working copy)
@@ -13,7 +13,8 @@
 	xmlns:content="http://purl.org/rss/1.0/modules/content/"
 	xmlns:dc="http://purl.org/dc/elements/1.1/"
 	xmlns:atom="http://www.w3.org/2005/Atom"
-	>
+	<?php do_action('commentrss2_ns'); ?>
+>
 <channel>
 	<title><?php
 		if ( is_singular() )