<div id="sub-menu">
<ul>
<?php if($post->post_parent)
$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0"); else
$children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
if ($children && is_page()) { ?>
<?php echo $children; ?>
<?php } else { ?>
<?php } ?>
</ul>
</div>