// Process the forms
if (($this->getRequest()->isPost())
        && ($this->getRequest()->isXmlHttpRequest())) {

    // Initiate response
    $status = false;
    $msg = '';
    $zf = null;

    // Test error
    $form->getElement('nobaru')->addError('This is the error message');

    if ($form->isValid($this->getRequest()->getPost())) {

    // Everything is good
        $status = true;

    } else {

    // Get the error messages
        $zf = $form->getMessages();
    }

    // Setup the response
    $result = json_encode(array('status' => $status,
                                'msg' => $msg,
                                'zf' => $zf));

    $this->_helper->viewRenderer->setNoRender();
    $this->_helper->layout()->disableLayout();

    $this->getResponse()->setHeader('Content-Type', 'application/json');
    $this->getResponse()->setBody($result);

    return;

} else {
    // Populate the form
}