Menu callback; regular HTML page with an X-XRDS-Location HTTP header.
function openid_test_yadis_x_xrds_location() {
drupal_add_http_header('X-XRDS-Location', url('openid-test/yadis/xrds', array(
'absolute' => TRUE,
)));
return t('This page includes an X-RDS-Location HTTP header containing the URL of an XRDS document.');
}