public function getFormKey()
    {
        if (Mage::registry('replace_form_key') &&
                ! Mage::app()->getRequest()->getParam('form_key', false)) {
            // flag request for ESI processing
            Mage::register('turpentine_esi_flag', true, true);
            return '{{form_key_esi_placeholder}}';
        } else {
            return parent::getFormKey();
        }
    }