an unofficial blog with some neat tips & tricks, including this dandy to get the Google PHP API client library working inside of your AppEngine app. This is huge because with the library, you can take advantage of some of Google's modern systems.
So now instead of using AppEngine's native User class, I can now use my OAuth login approach. The old (2007!) User class was ok, but you aren't able to do much with the resulting user, beyond getting their nickname and ID #. With OAuth, you can get their profile picture to slap into your header, ask them to integrate with their other services (which you can use the library for, too).
My guess is that Google's working towards getting the library integrated into the runtime, but there's probably some dust that needs to settle somewhere.