Form submit function. Update the role weights.
function user_admin_roles_order_submit($form, &$form_state) {
foreach ($form_state['values']['roles'] as $rid => $role_values) {
$role = $form['roles'][$rid]['#role'];
$role->weight = $role_values['weight'];
user_role_save($role);
}
drupal_set_message(t('The role settings have been updated.'));
}