There's always a degree of lock-in to any setup. That said, you can minimize lock-in by going with Django as much as you can and providing an abstraction layer

There's always a degree of lock-in to any setup. That said, you can minimize lock-in by going with Django as much as you can and providing an abstraction layer to whatever APIs you use. Open-source key-value stores are popping up everywhere, so non-Google-hosted App Engine frameworks will probably appear in the next year. (AppDrop provided a fast check on porting App Engine SDK to EC2.)