title: Django unique_together doesn't work with ForeignKey=None
body: <p>I saw some ppl had this problem before me, but on older versions of Django, and I'm running on 1.2.1.</p>
  
  <p>I have a model that looks like:</p>
  
  <pre><code>class Category(models.Model):
   objects = CategoryManager()
  
   name = models.CharField(max_length=30, blank=False, null=False)
   parent = models.ForeignKey('self', null=True, blank=True, help_text=_('The direct parent category.'))
  
   class Meta:
    unique_together = ('name', 'parent')
  </code></pre>
  
  <p>Whenever i try to save in the admin a category with a parent set to None, it still works when there's another category with the SAME name and parent set to None.</p>
  
  <p>Ideas on how to solve this gracefully?</p>