Form element #value_callback: assembles the allowed values for 'boolean' fields.
function list_boolean_allowed_values_callback($element, $input, $form_state) {
$on = drupal_array_get_nested_value($form_state['input'], $element['#on_parents']);
$off = drupal_array_get_nested_value($form_state['input'], $element['#off_parents']);
return array(
$off,
$on,
);
}