I'm surprised to see a suggestion of CouchDB - with the deep integration of BigTable with App Engine, I think it'd be unnecessarily hard to use anything else.
PageForest uses App Engine to provide a key value storage API with user authentication. CouchDB has these out-of-the-box, so a CouchDB version should be much less code than this version. Not to mention all the things Couch supports that would be a major challenge to implement within App Engine.
