As mentioned before, there is AppScale which will let you avoid the lock-in if you so desire. AppScale has been around for quite a while, and it has been sponsored by Google, but it is an independent company and an open source project. AppScale has been running customers application reliably for few years, with Datastore load of up to a quarter million of transactions (and this is an old white paper).

AppScale will allow you to move your application (unchanged) and still reap the benefits of the App Engine model, autoscaling and all. I still believe App Engine benefits trumps any other PaaS out there.