[Pat Fairbank]

I assume you're using Python?

I've run scripts before on App Engine that have created a few hundred objects in one go, and they seem to work okay (in the cloud, at least; it totally kills the local development server whose datastore implementation just can't compete with Bigtable).

I don't have any experience with this, but I'm pretty sure you can pass an optional RPC object argument that can specify a callback, to both the URL fetcher and the datastore, to make them act asynchronously. That way, you should be able to run operations simultaneously.