Index: wp-includes/comment-template.php
===================================================================
--- wp-includes/comment-template.php	(revision 19601)
+++ wp-includes/comment-template.php	(working copy)
@@ -1422,6 +1422,7 @@
 		'page' => '', 'per_page' => '', 'avatar_size' => 32, 'reverse_top_level' => null, 'reverse_children' => '');
 
 	$r = wp_parse_args( $args, $defaults );
+	$r = apply_filters( 'wp_list_comments_args', $r );
 
 	// Figure out what comments we'll be looping through ($_comments)
 	if ( null !== $comments ) {