According to eWeek, Google will make GAE open to public use on May 28th. Which would be today.
The original GAE site isn’t updated at this point, but you can get started anyway. I just set up my account and registered an app. (I predict tens of thousands of empty apps. Long-tail distribution here, just like SourceForge: an overwhelming majority of empty projects, with a vanishingly tiny minority that have 99% of the traffic.)
Now I just need to find time to learn Python and write something cool.