I've got same problem, but my Smarty is integrated a different way that yours.
Index.php
PHP Code:
include 'smarty/Smarty.class.php';
$smarty = new Smarty();
$smarty->debugging = false;
$smarty->force_compile = true;
$smarty->caching = false;
$smarty->compile_check = true;
$smarty->cache_lifetime = -1;
$smarty->template_dir = '../application/templates';
$smarty->compile_dir = '../application/templates_c';
$smarty->plugins_dir = array(
SMARTY_DIR . 'plugins',
'resources/plugins');
Zend_Registry::set('smarty', $smarty);
Controller
PHP Code:
$smarty = Zend_Registry::get('smarty');
$form = new forms_AddForm();
$smarty->assign('form', $form);
$smarty->display('user/add.tpl');