Update: Please star Issue 404 to bring OpenID to Google App Engine
Issue 404 is the key to getting OpenID working on Google App Engine but currently it only has 10 stars.
(Please add a star, _not_ a "me too" comment which will get emailed to everyone who has starred the issue and may cause other people to unstar it.)
Ryan Barrett from the Google App Engine team has built both OpenID provider and OpenID consumer libraries (open source) that you can reach at http://openid-provider.appspot.com/ and http://openid-consumer.appspot.com/ (uses version 2.1.1 of JanRain's OpenID library and is compatible with OpenID 2.0 providers).
Once this bug is fixed, those existing libraries will just work and we will have OpenID support on Google App Engine.
Please star Issue 404 so OpenID doesn't stay 404 on Google App Engine.