There's discussion of the Python version supported by AppEngine in there ... it's funny to think that once PyPy supports Python 3, and assuming the RPython interpreter implementing that support will still be compatible with Python 2.x, you might actually be able to use Python 3 on AppEngine by running PyPy's Python 3 interpreter against whatever Python 2.x AppEngine offers.

In fact, has anyone tried running PyPy's 2.7 interpreter on AppEngine's 2.5?

Not that I would expect such a setup to be production-ready, but just for the fun of it ...