$("document").ready(function(){
    $("#id_country").change(function(){
        country = $(this).val() ? $(this).val() : 0;
        if (country){
            update_region(country);
        } else {
            $("#id_region").html('<option value="0">- регион -</option>');
        }
    });
});

function update_region(country){
    $("#id_region").attr("disabled","disabled");
    $.ajax({
        type: "POST",
        url: "/catalog/ajax/region",
        data: "country="+country,
        success: function(html){
            if (html){
                $("#id_region").html(html).removeAttr("disabled");
            }
        }
    });
}