So after a day of mulling over GAE, I've come up with one conclusion. For one-off apps, this is brilliant. What Ruby and Rails did in it's heyday, this is going to do. No need to figure out how to set up a web server or configure a database. Just write some python and blamo an application.
This also blows Facebook applications out of the water. With GAE, you get a full web framework, written on top of a real language. You aren't forced to use FBML, you can use Django templates. Tie in opensocial and you have a turn-key application platform.
Now, I'm not sure that this is a viable solution to run a website on (what is a website anymore?), but writing a web application, it's brilliant. Next up, Google Gears for appengine.