class My_Form_Registration extends Zend_Form
{
    // ...
    
    /**
     * Is the sub form valid?
     * 
     * @param  Zend_Form_SubForm $subForm 
     * @param  array $data 
     * @return bool
     */
    public function subFormIsValid(Zend_Form_SubForm $subForm,
                                   array $data)
    {
        $name = $subForm->getName();
        if ($subForm->isValid($data)) {
            $this->getSessionNamespace()->$name = $subForm->getValues();
            return true;
        }

        return false;
    }

    /**
     * Is the full form valid?
     * 
     * @return bool
     */
    public function formIsValid()
    {
        $data = array();
        foreach ($this->getSessionNamespace() as $key => $info) {
            $data[$key] = $info;
        }

        return $this->getForm()->isValid($data);
    }
}

class RegistrationController extends Zend_Controller_Action
{
    // class contents...