$form = new Zend_Form();
$group = new Zend_Form_Element_MultiCheckbox('groups');
$group->addMultiOption(1,'A');
$group->addMultiOption(2,'B');
$group->addMultiOption(3,'C');

// check item 1
$group->setValue(1);

// check items 1 and 3
$group->setValue(array(1,3));

// check all items
$group->setValue(true);


$form->addElement($group);

echo $form->render();


ciao Marcello