I was wondering about this in October, I guess the Vosao CMS needs to stop storing state in static fields now:

http://stackoverflow.com/questions/4028787/is-it-thread-safe...

Thilo added answer a few hours ago, pointing out the new threadsafe mode for GAE. That's why I like StackOverflow.

Your code only needs to be threadsafe if you wish to enable the concurrent flag, otherwise your code will happily run as it had before.

I just thought it's important to note that they're not forcing you to push a code update unless you want to handle concurrent requests with a single instance.