I setup a Zend Form object and needed radio buttons, but also needed a default value. Took me a few minutes to find, but here’s an example of use.

/**
 * Setting some values for our radio buttons.
 */
$options = array(
    '1' => 'Option 1',
    '2' => 'Option 2',
    '3' => 'Option 3'
);

/**
 * The array('value' => 2) below sets our default value.
 */
$radio = new Zend_Form_Element_Radio('elementName', array('value' => '2'));
$radio->addMultiOptions($options);
$this->addElement($radio);