Here is some code added to shipping_estimator.php for 4.3 used with
Yahoo PlaceFinder that might point you in the right direction:
PHP Code:
$defState = 'AK';
require_once('PlaceFinder_api.php');
$ypl = new yahoo_placeFinder();
$ypl->postal = $zipcode_estimate;
$ypl->country = $default_country_estimate;
$ypl->addFlag('PHP');
$yplData = unserialize($ypl->execute());
$defState = $yplData['ResultSet']['Result'][0]['statecode'];