Google App Engine compatibility layer
If you develop with web2py your code will run GAE other architectures wihtout changes using any of the 10 supported relational databases. The compatibility layer covers database api (including blobs and listproperty), email, and fetching).
I'm planning an application running on Google App Engine. The only worry I would have is portability. Or just the option to have the app run on a local, private cluster. 
I expected an option for Google App Engine applications to run on other systems, a compatibility layer, to spring up. I could imagine a GAE compatible framework utilizing Amazon SimpleDB or CouchDB to offer near 100% compatibility, if needs be through an abstraction layer. I prefer Python though Java would be acceptable.
However, as far as I know, none such facility exists today. Am I mistaken and if so where could I find this Googe App Engine compatibility layer. If I'm not, the questions is "why"? Are there unforetold technical issues or is there just no demand from the market (which would potentially hint at low rates of GAE adoption).
Regards,
Iwan