Admin top level menu, pointing to an external url

Just add your custom item into $menu instead of $submenu.

Use here as a reference for parent menu structure.
http://core.trac.wordpress.org/browser/tags/3.2.1/wp-admin/menu.php

Eg.

add_action( 'admin_menu' , 'admin_menu_wpse32975' );
function admin_menu_wpse32975() {
    global $menu;
    $menu[9999] = array( __('My Menu'), 'manage_options', 'http://www.google.com', '', 'open-if-no-js menu-top', '', 'div' );
}