How-to: Alter nav menu output (in this case adding blueprint css classes)

nav-menus.php

<?php
/**
 * Classes for a navigation named "Topnav" in the nav location "top".
 * 
 * You will have to re-create this function inside your Child Theme.
 * Just edit the data - "editing" comments - inside your Child Themes function. 
 * 
 * @since	0.1
 * @since	0.2 - replaced by the new function that saves 24(!) queries.
 * 
 * @param (object) $items
 * @param (object) $menu
 * @param (object) $args
 * @return (object) $items
 */
function oxo_nav_top_classes( $items, $menu, $args )
{
        // avoid destroying the admin UI
        if ( is_admin() )
              return;

	# >>>> start editing

	// Nav menu name you entered in the admin-UI > Appearance > Menus (Add menu).
	$target['name'] = 'Topnav';
	// The targeted menu item/s
	$target['items'] = array( (int) 6 );

	# <<<< stop editing

	// filter for child themes: "config_nav_menu_topnav"
	$target = apply_filters( 'config_nav_menu_'.strtolower( $target['name'] ), $target );

	// Abort if we're not with the named menu
	if ( $menu->name !== $target['name'] ) 
		return;

	foreach ( $items as $item )
	{
		// Add the class for each menu item
		$item->classes = 'span-4';

		// Append this class if we are in one of the targeted items
		if ( in_array( (int) $item->menu_order, $target['items'] ) )
			$item->classes .= ' last';
	}

	return $items;
}
add_filter( 'wp_get_nav_menu_items', 'oxo_nav_top_classes', 10, 3 );
?>