Challenge accepted: See my latest post, which explains how jython can do something on Google AppEngine that cpython cannot do: XSLT transforms.

Transforming with XSLT on Google AppEngine and jython.
http://jython.xhaus.com/transforming-with-xslt-on-google-appengine-and-jython/

I’m looking forward to seeing anyone doing the same with cpython on AppEngine

I think you’re misconstruing my comments at the event. While I can’t recall how I phrased it exactly, the gist of what I intended to say was that while it was possible to run Jython on App Engine, I couldn’t think why you’d want to. I didn’t mean to imply that I was certain it wasn’t ever useful.

The use-case I described after the talk was one I’d encountered where a user wanted to port his Python app to Java (or vice-versa) progressively, without interrupting service – not a case of prototyping, but rather migration.

I also think your example of cryptography libraries for library support is perhaps not the best one: It’s especially important in cryptography not to have a profusion of libraries, but rather to have at least one really good one. KeyCzar (http://www.keyczar.org/) is, in my opinion at least, an example of such a library – and it’s available natively for both Python and Java.

Please don’t take these corrections as criticism of the concept of Jython on App Engine, though. I love to see novel uses of App Engine and of programming languages in general. I’m sure an article on how to write a basic webapp in Jython on App Engine would be very well received – as would further practical examples of using the two together.