I ran across a module to create site sections somewhere... but needed a quick solution for a single block. I wanted the block to show up if and only if we're on a page which is in the menu structure as a child of the first primary link. Enter "Show if the following PHP code returns TRUE" and voila:
<?php
$tab1 = array_shift(menu_tree_page_data('primary-links'));
return $tab1['link']['in_active_trail'];
?>