--- system.admin.inc	Thu Jan 15 10:14:12 1970
+++ system.admin.inc	Thu Jan 15 10:14:12 1970
@@ -318,6 +318,10 @@
   // path after that.
   module_invoke('locale', 'system_update', array_diff($new_theme_list, $old_theme_list));
 
+  // make sure that later calls to _system_theme_data
+  // don't get cached theme information.
+  _system_theme_data(TRUE, TRUE);
+
   return;
 }
 
--- system.module	Thu Jan 15 10:14:12 1970
+++ system.module	Thu Jan 15 10:14:12 1970
@@ -828,12 +828,27 @@
 /**
  * Helper function to scan and collect theme .info data and their engines.
  *
+ * @param $reset :bool
+ *   if TRUE, the cached value will be reset
+ * 
+ * @param $skip_run :bool
+ *   if TRUE, the function will skip the run.
+ *   makes sense in combination with $reset=true, if we want
+ *   to make sure that later calls don't get a cached result,
+ *   but we are not interested in the return value.
+ *  
  * @return
  *   An associative array of themes information.
  */
-function _system_theme_data() {
+function _system_theme_data($reset = FALSE, $skip_run = FALSE) {
   static $themes_info = array();
 
+  if ($reset) {
+    $themes_info = array();
+  }
+  if ($skip_run) {
+    return;
+  }
   if (empty($themes_info)) {
     // Find themes
     $themes = drupal_system_listing('\.info$', 'themes');