Get the top parent id http://www.webcitizenmag.com/2010/05/20/how-to-get-top-parent-page-id-in-wordpress/
<?php
function get_top_parent_page_id() {
global $post;
$ancestors = $post->ancestors;
// Check if page is a child page (any level)
if ($ancestors) {
// Grab the ID of top-level page from the tree
return end($ancestors);
} else {
// Page is the top level, so use it's own id
return $post->ID;
}
}
?>