Found this today: http://code.google.com/p/gmemsess/
gmemsess is a secure lightweight memcache-backed session class for Google appengine. Currently it is only suitable for short-term sessions, for providing your own authentication system or shopping cart, for instance. The session cookie expires when the browser is closed, and the reliability of memcache for longer-term storage has yet to be demonstrated.Pretty nice. I guess it should have a datastore backup, though, since we don't know how the MemCache API evicts data. Still cool that the little open source projects plop up so fast!