Those who are talking about being 'locked in' to App Engine, there are solutions, like http://www.datastorebackup.com

The essence of your data is saved. Nothing to worry about then..

And regarding the pricing... We'll see.. I'm not sure WHEN exactly it's going to be shifted - does anyone know?

It's not only about the data, however. It's also about the x lines of code that are GAE specific. The datastore API is non-standard, and cannot easily be ported elsewhere.