Just read that Appcelerator has been ported to Google’s new App Engine. I had never run across Appcelerator before in my searching for Python web related libraries. Perhaps because Appcelerator supports a number of languages other then Python (Java, PHP, Ruby, .NET, and Perl).
Since Appcelerator has been ported to Google’s new App Engine it may allow one to write a web app such that it can be run both on App Engine and on one’s own servers (or in the future other folks cloud’s). Whether that is feasible or not will depend on how much of App Engine is exposed through Appcelerator. Right now the Appcelerator site has numerous messages about new documentation being in the process of being pushed to the site so once their docs are updated the picture will be come clearer.