Messages when performing operations are incorrect and inconsistent

    if ($entity_type->isNew) {
      drupal_set_message(t('Entity type %entity_type has been created.', array('%entity_type' => $entity_type->label)));
    }
    else {
      drupal_set_message(t('Entity type %entity_type has been updated.', array('%entity_type' => $entity_type->label)));
    }

There is actually this code there already, but $entity_type->isNew gets wiped when the entity type is saved a few lines earlier...

I'm not sure how best to fix it.

I'd be inclined to say that isNew shouldn't get wiped by save(), but then I don't know what else might rely on it. Also, I feel really wary of this:

  protected function __construct($table) {
    $this->serialize = array();
    $this->isNew = TRUE;

because that means that the flag gets set even when an entity type object is being loaded from the database, when it's surely not new!