see bug http://drupal.org/node/921686

the feedburner module makes implicit calls to url_alter before implementations of url_alter are loaded:

/**
* Implementation of hook_boot().
*/
function feedburner_boot() {
drupal_bootstrap(DRUPAL_BOOTSTRAP_PATH);
$path = _feedburner_get_path_segment();
feedburner_check_redirect($path);
}

this breaks implementations that use url_alter.

Gerd