Currently, dev-php/smarty installs into /usr/lib/php/Smarty while /usr/lib/php/smarty would suit a lot better. Reason for this is Smarty's own path auto-detection. Suppose I /don't/ define SMARTY_DIR and just "require_once 'Smarty/Smarty.class.php' (the default installation directory). This will fail, because Smarty can't guess it's own plugin path. Now, I rename /usr/lib/php/Smarty to /usr/lib/php/smarty and "require_once 'smarty/Smarty.class.php'. Result: success. Actually, the ports system on FreeBSD just does this.
Fixed in smarty-2.6.12, thanks for reporting it.