<?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) {  
    echo '<ul>';  
        echo $children;  
    echo '</ul>';  
} ?>