when someone have the same problem: you should write your own session handler in php, because the session handler that implemented on php can not deal with Asynchronous JavaScript and XML. On simultanious request sessions can be overritten. Maybe you should serialize your session handler. On a file-based Session handler is always a serial handling! You should think about to do the same with your Memcached Session-Handler!
http://de2.php.net/manual/de/function.session-set-save-handler.php
