<?php if ( is_page() ) {
if($post->post_parent)
$children = wp_list_pages('sort_column=menu_order&title_li=&child_of='.$post->post_parent.'&echo=0');
else
$children = wp_list_pages('sort_column=menu_order&title_li=&child_of='.$post->ID.'&echo=0');
if ($children) {
?>
<div class="sidebar">
<h2>Sub-pages of Current Page</h2>
<ul>
<?php echo $children; ?>
</ul>
</div>
<?php
} // End If Post
} // End if is page
?>