I can't wait to read the source code of this. I think it will be a great way to learn development and design on App Engine.
Shameless plug: not that it's at the same scale, but I put together the game of Go as an AppEngine app. You can play it at http://go.davepeck.org/ or read the AGPLv3 sources at http://github.com/davepeck/appengine-go/
(I wrote "Go" to share with the audience of a talk I gave about scaling with AppEngine; I do a lot of AppEngine and AWS work for my clients but alas can't share that work.)