$fieldset->addField('store_id', 'multiselect', array(
    'name'      => 'store_id',
    'label'     => Mage::helper('my_custom_widget_extended')->__('Store View'),
    'title'     => Mage::helper('my_custom_widget_extended')->__('Store View'),
    'required'  => true,
    'values'    => Mage::getSingleton('adminhtml/system_store')->getStoreValuesForForm(false, true),
));