We use Appengine, which only supports python 2.7. As well as being a reason for us to stick with 2.7, I imagine Google prefers what is effectively a LTS release. If (hopefully "when") they provide 3.x, they will need significantly more support resources to keep up with continual releases.
Thanks for correcting me - after excitedly checking the docs, the "flexible" service indeed supports 3.0. The managed service is still 2.7, however.
The flexible service is basically running your own instance of appengine on a VPS, which means you have to provision and support the infrastructure. To my mind, that negates most of the advantage of Appengine - if I was happy to do my own system admin, I'd build my own stack on VPS.
