Wow. I'm not a GAE user, so I'm not embarrassed that I didn't know this. However, I'm surprised that such a restriction hasn't come up in all the blog entries I've read about GAE over the years.
It's not a huge restriction since GAE will happily spin up dozens of instances without any intervention if your load requires it. Getting multiple concurrent requests into each instance will make things like local memory caching somewhat more useful though.