In Rails, I believe the database backend can be either. Look at Twitter - their using Cassandra.
Cursebird (http://cursebird.com/) is powered by Mongo via MongoEngine (http://mongoengine.org/) and Redis via redis-py (http://github.com/andymccurdy/redis-py). None of the Django apps have a models module.
Google actually gave up those efforts.
Also, the Django ORM is pretty flexibile, and the architecture just got improvements for NoSQL in 1.2.
There was a Django patch somewhere that used a thin layer to make the API "look" like the Django ORM, but IMHO it is broken in a lot of ways and you're better off using the raw App Engine API (which is in no way related to Django).
But it is a half-ass attempt, looking like some patch somebody did in an afternoon to get something working: and that was the "Google effort"?