Index: wp-includes/comment.php
===================================================================
--- wp-includes/comment.php	(revision 22074)
+++ wp-includes/comment.php	(working copy)
@@ -1293,6 +1293,8 @@
 	$comment = get_comment($id);
 	do_action('wp_insert_comment', $id, $comment);
 
+	wp_cache_set('last_changed', time(), 'comment');
+
 	return $id;
 }