We just open sourced GAE/Bingo, a re-imagining of patio11's A/Bingo for use in App Engine (w/ his approval). The lib was built for Khan Academy, where it's currently serving up A/B tests at a few hundred requests/second w/ minimal page load drag.
It should get anyone on App Engine (python) up and running and A/B testing in minutes -- if you have trouble please let me know. Does its best to maintain identities as users transition from anonymous to logged-in, persists experiment results to the datastore in the background, etc.
http://bjk5.com/post/10171483254/a-bingo-split-testing-now-on-app-engine-built-for-khan
https://github.com/kamens/gae_bingo
Feedback/patches welcome