add_action for saving a custom menu

At least in 3.4.1, there is an action for that: wp_update_nav_menu

See here.

Then you can get the items in your menu with something like:

add_action('wp_update_nav_menu', 'my_get_menu_items');
function my_get_menu_items($nav_menu_selected_id) {
    $items = wp_get_nav_menu_items($nav_menu_selected_id);
}