cidades_estados.js

$(document).ready(function(){

    $("#id_state").change(function(){
        carrega_cidades(0);
    });

    function carrega_cidades(id_selected) {
        var arr;
        $.get('/get_cidades/'+$("#id_state").val()+'/', function(data,status)
        {
            $('#id_city option').remove();
            for (i in data) {
                var obj = data[i];
                if (id_selected == obj.id)
                    $("#id_city").append($('<option>').attr('value', obj.id).attr('text', obj.title).attr('selected', true).html(obj.title));
                else
                    $("#id_city").append($('<option>').attr('value', obj.id).attr('text', obj.title).html(obj.title));
            }
        });
    }

});

forms.py

class MeuForm(forms.Form):
    nome = forms.CharField(label='* Nome', max_length=255, required=True,)
    state = forms.ModelChoiceField(queryset=State.objects.all(), label='Estado',)
    city = forms.ModelChoiceField(queryset=City.objects.all(), label="Cidade",)

    def __init__(self, *args, **kwargs):
        super(MeuForm, self).__init__(*args, **kwargs)
        self.fields['city'].choices = [('', '---------')]

models.py

class City(models.Model):
    id = models.IntegerField(primary_key=True)
    id_state = models.ForeignKey('State', db_column='id_state')
    title = models.CharField(max_length=50)
    iso = models.IntegerField()
    iso_ddd = models.CharField(max_length=6)
    created_at = models.DateTimeField(blank=True, null=True)
    updated_at = models.DateTimeField(blank=True, null=True)
    status = models.CharField(max_length=1)

    def __unicode__(self):
        return "%s" % self.title

    class Meta:
        managed = False
        db_table = 'city'
        
        
class State(models.Model):
    id = models.IntegerField(primary_key=True)
    id_country = models.ForeignKey(Country, db_column='id_country')
    id_region = models.IntegerField()
    title = models.CharField(max_length=35)
    letter = models.CharField(max_length=2)
    iso = models.IntegerField()
    created_at = models.DateTimeField(blank=True, null=True)
    updated_at = models.DateTimeField(blank=True, null=True)
    status = models.CharField(max_length=1)

    def __unicode__(self):
        return "%s" % self.letter

    class Meta:
        managed = False
        db_table = 'state'

views.py

def get_cidades(request, id_estado):
    if id_estado:
        cidades = City.objects.filter(id_state=id_estado)
    else:
        cidades = []
    response_data = [{'id': '', 'title': '---------'}]
    for c in cidades:
        response_data.append({'id': c.id, 'title': c.title})

    return HttpResponse(
        json.dumps(response_data),
        content_type="application/json"
    )