by Dave Reid: Front page path is always normalized in D7.

xmlsitemap.module

@@ -173,7 +173,7 @@ function xmlsitemap_xmlsitemap_links() {
  */
 function xmlsitemap_xmlsitemap_link_alter(&$link) {
   // Alter the frontpage priority.
-  if ($link['type'] == 'frontpage' || $link['loc'] == '' || $link['loc'] == drupal_get_normal_path(variable_get('site_frontpage', 'node'))) {
+  if ($link['type'] == 'frontpage' || $link['loc'] == '' || $link['loc'] == variable_get('site_frontpage', 'node')) {
     $link['priority'] = xmlsitemap_var('frontpage_priority');
     $link['changefreq'] = xmlsitemap_var('frontpage_changefreq');
   }