[knuckle_cake]

While I don't have enough Django experience to give suggestions on that front, you can use BigTable if you want to move to Google App Engine, though the drawbacks may outweigh the benefits when dealing with an established app.

That said, I did recently make this choice for a Ruby/Sinatra app I'm working on, and ended up going with MongoDB due to conveniences like MongoMapper more than anything else (I wanted to enforce a partial schema without having to resort to silliness like customField1, customField2, etc.) I'm pleased with this choice so far, though plan to look at Redis again in the future when I need something lighter in weight for storage.