AppScale, an OpenSource GAE implementation
AppScale might change that.
AppScale is an open-source implementation of the Google AppEngine (GAE) cloud computing interface from the RACELab at UC Santa Barbara. AppScale enables execution of GAE applications on virtualized cluster systems. In particular, AppScale enables users to execute GAE applications using their own clusters with greater scalability and reliability than the GAE SDK provides. Moreover, AppScale executes automatically and transparently over cloud infrastructures such as the Amazon Web Services (AWS) Elastic Compute Cloud (EC2) and Eucalyptus, the open-source implementation of the AWS interfaces.
The list of supported infrastructures is very impressive. However the key, in my personal opinion, would be stability and compatibility with current GAE APIs.
Learn more about AppScale: