In parallel we can find severals open source backend implementations of google backend.
So now we can use these applications or develop new ones hosted in a private backend.
So why don't we use the gae sdk as a framework to build portable applications.
In fact gae standart sdk propose two frameworks, webapp and django, and we can find more open source frameworks based on gae sdk: webpy, gaeo.
Thus we could speak about a framework of frameworks - the common part is the sand box system of gae and its API (urlfetch, auth, mail, etc.) based on google protobuf.
So this site offers a compilation of useful links to build your own Google App Engine, frontend and backend.
Let's dive into Paas, IaaS, Cloud computing, utility computing and all this cloud computing taxonomy.