But you could quickly recover by hosting those JS files yourself and relinking them. If your app is coupled tightly with AppEngine APIs, then there is nowhere else you can host your app.

Theoretically you could fire up AppScale on EC2: http://code.google.com/p/appscale/

What about your data? Most web applications have persistent data of some sort that is vital to the user experience -- without it, you don't have much of a site.

[Edit: And keeping a hot copy of that data is a lot harder than it sounds]

GAE still had read-only access to data so it would be possible to backup and move elsewhere