I just enabled billing just because my daily bandwidth seemed to exceed 1 GB per day.
But I realized from the Billing History section, days before the day the billing is enabled, the daily bandwidth has exceeded 1 GB (even it ever reached 2.5 GB, and the last column says $0.18), and I was not charged anything.
How come the free version of app engine allows more than 1 GB of bandwidth? If it is so, then there is no purpose of enabling billing.