Thanks for the info. Is there a setting for max number of simultaneous instances? If your app gets slashdotted are your potential costs completely unbounded?
You won't, because you specify a maximum daily spend in the budget settings. So when you get slashdotted the scheduler will spin up X hundred instances, your budget will run out in 1/2 hour, and your site will be down for anything up to the next 23 1/2 hours.
(Just out of curiosity, I'm not a GAE customer)