Actualy I'm not sure if this is bug or feature, but I'd expect all elements extending Zend_Form_Element_Multi to work as enums.

Having this element:

$element = new Zend_Form_Element_Select('name');
$element->addMultiOption('foo', "FOO");
$element->addMultiOption('bar', "BAR");

Expected behaviour:

$element->isValid('foo'); // returns true
$element->getValue();     // returns "foo"

But I was a bit surprised that a value which is unlisted can be accepted and returned:

$element->isValid('unlisted-value'); // returns true
$element->getValue(); // returns "unlisted-value"

I know this can be solved by adding InArray validator. But I would expect it's (probably the main) purpose and default behaviour of these elements not to let through unavailable options. If this behaviour is realy so intended, I think there should be at least warning in manual that it's upon to developer to provide the check manually. This relates to Select, MultiSelect, MultiCheckbox and Radio elements.