class Cliente(models.Model):
    user = models.ForeignKey(User, editable=False)
    password = models.CharField(max_length=30)
    ...

    class Meta:
        ordering = ['-id']

    def save(self):
        if not self.id:
            c = Cliente.objects.filter(email=self.email).count()
            if c:
                raise EmailExistente

            usr = User.objects.filter(username=self.email)
            if usr:
                u = usr[0]
            else:
                u = User.objects.create_user(self.email, self.email, self.password)
            u.save()
            self.user = u
        else:
            self.user.username = self.email
            self.user.email = self.email
            self.user.set_password(self.password)
            self.user.save()

        super(Cliente, self).save()

----------------------------------------------------------------------------------------------
class FormCliente(forms.ModelForm):

    password = forms.CharField(widget=forms.PasswordInput(render_value=True))
    password_confirm = forms.CharField(widget=forms.PasswordInput(render_value=True))

    class Meta:
        model = Cliente


    def clean_nome(self):
        return re.sub('[.,/-]', '', unicode(self.cleaned_data['nome']))

    def clean_sobrenome(self):
        return re.sub('[.,/-]', '', unicode(self.cleaned_data['sobrenome']))

    def clean_password(self):
        if self.cleaned_data.get('password', '') != '':
            if len(self.cleaned_data.get('password')) < 6:
                raise forms.ValidationError(u'A password deverá ter, no mínimo, 6 caracteres!')
        return self.cleaned_data['password']

    def clean_password_confirm(self):
        if self.cleaned_data.get('password', '') != '':
            if self.cleaned_data.get('password', '') != self.cleaned_data.get('password_confirm', ''):
                raise forms.ValidationError(u'As passwords não conferem.')
        return self.cleaned_data['password_confirm']

----------------------------------------------------------------------------------