Index: wp-includes/comment-template.php
===================================================================
--- wp-includes/comment-template.php	(revision 13622)
+++ wp-includes/comment-template.php	(working copy)
@@ -1496,9 +1496,11 @@
 						'title_reply_to' => __( 'Leave a Reply to %s' ),
 						'cancel_reply_link' => __( 'Cancel reply' ),
 						'label_submit' => __( 'Post Comment' ),
+						'echo' => true,
 				);
 	$args = wp_parse_args( $args, apply_filters( 'comment_form_defaults', $defaults ) );
-
+	
+	ob_start();
 	?>
 		<?php if ( comments_open() ) : ?>
 			<?php do_action( 'comment_form_before' ); ?>
@@ -1538,6 +1540,11 @@
 			<?php do_action( 'comment_form_comments_closed' ); ?>
 		<?php endif; ?>
 	<?php
+	$form =	apply_filters( 'comment_form_output', ob_get_clean(), $args);
+	if ( $args['echo'] )
+		echo $form;
+	else
+		return $form;
 }
 
 ?>