The callback validation in the callback decorator is too restrictive and arguably wrong.

    public function setCallback($callback)
    {
        if (!is_string($callback) && !is_array($callback)) {
            require_once 'Zend/Form/Exception.php';
            throw new Zend_Form_Exception('Invalid callback provided to callback decorator');
        }

        if (is_array($callback)) {
            if (2 !== count($callback)) {
                require_once 'Zend/Form/Exception.php';
                throw new Zend_Form_Exception('Invalid method callback provided to callback decorator');
            }
        }

        $this->_callback = $callback;
        return $this;
    }

That whole check should simply be {{if (!is_callable($callback)) {throw new Zend_Form_Exception();}}}. As it is right now, it won't accept the return values of create_function or PHP 5.3-style lambdas.