Index: wp-includes/comment.php
===================================================================
--- wp-includes/comment.php	(revision 10475)
+++ wp-includes/comment.php	(working copy)
@@ -739,6 +739,8 @@
 	if ( ! $wpdb->query( $wpdb->prepare("DELETE FROM $wpdb->comments WHERE comment_ID = %d LIMIT 1", $comment_id) ) )
 		return false;
 
+	$wpdb->update($wpdb->comments, array('comment_parent' => $comment->comment_parent), array('comment_parent' => $comment_id));
+
 	$post_id = $comment->comment_post_ID;
 	if ( $post_id && $comment->comment_approved == 1 )
 		wp_update_comment_count($post_id);