Zend_View_Helper_FormSelect does not allow optgroup elements to have an id attribute. Here is a patch that should allow for the following use case but should still be backwards compatible. This is not necessarily the best way to do this and could probably be a lot better: I put it together very quickly. The existing code doesn't seem to follow the Zend Framework coding standards so I kept my new code similar to the code that's already there. Again, this could be a lot more extensible than it is (e.g. it only allows for id and not class or other arbitrary decorators).

Here is the desired output:

Option AOption BOption COption D

Here is the use case that will give the desired output (with the patch applied):

$optgroupTest = new Zend_Form_Element_Select('optgroup_test');
$options = array (
    array(
        'id'    => 'option_group_1',
        'label' => 'Option Group 1',
        'options' => array (
            'A'     => 'Option A',
            'B'     => 'Option B',
            'C'     => 'Option C',
        ),
    ),
    array(
        'id'    => 'option_group_2',
        'label' => 'Option Group 2',
        'options' => array (
            'D'     => 'Option D',
        ),
    ),
);
$optgroupTest
    ->setLabel('Optgroup Test')
    ->addMultiOptions(
        $options
    )
;