<?php
/* Useful, for example, for showing "tabs" at the top of a page which show all children of that page */

  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&depth=1");
  if ($children) { ?>
  <ul>
  <?php echo $children; ?>
  </ul>
<?php } ?>