Using dojo/json to check for username availability

<script language="JavaScript" type="text/javascript">
    dojo.require("dojo.io.*");
    dojo.require("dojo.json");

        function sendFormCallback(type, data, evt) {
            if (type == 'error')
                alert('Error when retrieving data from the server!');
            else
                arrayData = dojo.json.evalJson(data);
                        // returned data from this critter will be an array with two
                        // keys - "username" and "available". i.e.
                        // {"username": "asdg", "available": true}
                        if (arrayData["available"])
                                dojo.byId("check_available_results").innerHTML = "Available"
                        else
                                dojo.byId("check_available_results").innerHTML = "Not Available"
        }

        function checkLinkInvoke() {
                username_input = dojo.byId('id_username')
                dojo.io.bind({
                        url: "/account/isavailable/",
                        method: "post",
                            /* optional content. If the content is in the
                             * form of a hashref they are converted to
                             * post paramters */
                            content: {
                                username: username_input.value,
                                        xhr: 1
                            },
                            load: sendFormCallback
                        });
        }
    function init() {
                var check_link = dojo.byId('checklink');
                check_link.style.display = "block"; // enables the check link
                dojo.event.connect(check_link, 'onclick', 'checkLinkInvoke');
        }

    dojo.addOnLoad(init);
</script>

and the associated view:

def available(request):
    """Return the availability status of a desired username.

    This is used in conjunction with dojo, on the account registration form.
    """
    if not request.POST:
        return HttpResponseRedirect(ACCOUNT_REGISTER_URL)

    if request.POST.has_key('xhr'):
        username = request.POST.get('username', None)
        response_dict={}
        response_dict['username'] = username
        response_dict['available'] = False
        if User.objects.filter(username=username).count() == 0:
            response_dict['available']=True
        return HttpResponse(simplejson.dumps(response_dict), mimetype='application/javascript')
    else:
        return HttpResponseRedirect(ACCOUNT_REGISTER_URL)