If I already know basic PHP/MySQL, what else should I learn to use Google App Engine?

I am not an expert, I just started reading up on this myself, but here are a couple of starting points on data storage.

Rather than a separately maintained SQL database, with App Engine you can define your dataset within the application, and Google handles the back end. It uses a SQL-like language called GQL.

”Mastering the datastore
Unlike most other Java web application development environments, App Engine’s data storage service is not based on a relational database management system. Instead, App Engine’s datastore (Python | Java) is built on top of Bigtable, and this will likely require you to re-think the methods that you have traditionally used when modeling and querying for data”

http://code.google.com/appengine/articles/datastore/overview.html

http://code.google.com/appengine/docs/python/datastore/overview.html

I forgot to add – here’s a simple App Engine tutorial, a quick view of how to create and upload an application.

Create a Free Proxy Server with Google App Engine
http://www.labnol.org/internet/setup-proxy-server/12890/

I wouldn’t say that Python is a big leap, but the syntax is completely different and will definitely take some getting used to. I recommend reading at least a couple Python tutorials and getting used to the language a little bit.