Index: wp-includes/comment-template.php
===================================================================
--- wp-includes/comment-template.php	(revision 9808)
+++ wp-includes/comment-template.php	(working copy)
@@ -1168,7 +1168,7 @@
 			$add_below = 'div-comment';
 		}
 ?>
-		<<?php echo $tag ?> <?php comment_class() ?> id="comment-<?php comment_ID() ?>">
+		<<?php echo $tag ?> <?php comment_class(empty( $args['has_children'] ) ? '' : 'parent') ?> id="comment-<?php comment_ID() ?>">
 		<?php if ( 'ul' == $args['style'] ) : ?>
 		<div id="div-comment-<?php comment_ID() ?>">
 		<?php endif; ?>
Index: wp-includes/classes.php
===================================================================
--- wp-includes/classes.php	(revision 9808)
+++ wp-includes/classes.php	(working copy)
@@ -822,6 +822,8 @@
 		$id_field = $this->db_fields['id'];
 
 		//display this element
+		if ( ! empty( $args[0] ) )
+			$args[0]['has_children'] = ! empty( $children_elements[$element->$id_field] );
 		$cb_args = array_merge( array(&$output, $element, $depth), $args);
 		call_user_func_array(array(&$this, 'start_el'), $cb_args);