adminFormFieldDependency.php

$form = new Varien_Data_Form();

$form->addField('yesno', 'select', array(
    'label'  => $this->__('Yes or No?'),
    'values' => Mage::model('adminhtml/system_config_source_yesnocustom')
        ->toOptionArray(),
));
$form->addField('custom_value', text, array(
    'label'  => $this->__('Other'),
));

// Append dependency javascript
$this->setChild('form_after', $this->getLayout()
    ->createBlock('adminhtml/widget_form_element_dependence')
        ->addFieldMap('yesno', 'yesno')
        ->addFieldMap('custom_value', 'custom_value')
        ->addFieldDependence('custom_value', 'yesno', 2) // 2 = 'Specified'
);