Under some circumstances it would be great if you could force WordPress to highlight a certain menu trail (down to a specified menu item) and let it do the magic as it usually does (assigning current- classes to appropriate menu items).
Assuming custom menu structure below I imagine I could do this like so:
Page 1
|- Item 1.1
|- Item 1.1.1
|- Item 1.2
Page 2
Page 3
<?php
wp_nav_menu(
array('active_trail_item' => wp_get_menu_item_by_id( 21 ))
);
?>
Besides wp_get_menu_item_by_id users could also find such function useful wp_get_menu_item_by_name.