Index: wp-blog-header.php
===================================================================
--- wp-blog-header.php	(revision 3088)
+++ wp-blog-header.php	(working copy)
@@ -118,7 +118,7 @@
 	@header('Content-type: ' . get_option('html_type') . '; charset=' . get_option('blog_charset'));
 } else {
 	// We're showing a feed, so WP is indeed the only thing that last changed
-	if ( $withcomments )
+	if ( $withcomments || $feed == 'comments-rss2' )
 		$wp_last_modified = mysql2date('D, d M Y H:i:s', get_lastcommentmodified('GMT'), 0).' GMT';
 	else 
 		$wp_last_modified = mysql2date('D, d M Y H:i:s', get_lastpostmodified('GMT'), 0).' GMT';