Usando o seguinte código:

class Organization(models.Model):
    name = models.CharField(max_length="100",)
    alias = models.SlugField()
    ...

class Division(Organization):
    parent_org = models.ForeignKey(Organization)

    class Meta:
        unique_together=['parent_org', 'alias']
        ...

Tentando syncdb me dê este erro:

Error: One or more models did not validate:
organizations.division: "unique_together" refers to alias. This is not in the 
same model as the unique_together statement.

Qualquer ajuda é apreciada,

Obrigado,

Eric