Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.933
diff -u -p -r1.933 system.module
--- modules/system/system.module	20 May 2010 08:47:00 -0000	1.933
+++ modules/system/system.module	11 Jun 2010 01:26:01 -0000
@@ -2840,6 +2840,9 @@ function system_cron() {
     ))
     ->condition('expire', REQUEST_TIME, '<')
     ->execute();
+
+  // Clean up expired sessions (for systems that have automatic session garbage collection off in php.ini)
+  _drupal_session_garbage_collection(ini_get('session.gc_maxlifetime'));
 }
 
 /**