appengine is a platform, how would open sourcing it help you? You have a spare datacenter? FWIW there is an opensource clone out there in the wild but you need a cloud infrastructure to run it and it's not up to date with the latest SDK.
Releasing it would allow others to run "appengines" so you could move from one provider to another as you can with "traditional" hosting. Appengine pretty much locks you in. I'm interested to see where OpenShift will lead though. https://openshift.redhat.com/
