This came as a bit of a surprise to me:

To protect your information from these unusual bugs, we also back it up to tape. Since the tapes are offline, they’re protected from such software bugs. But restoring data from them also takes longer than transferring your requests to another data center, which is why it’s taken us hours to get the email back instead of milliseconds.

I had really not expected them to keep offline backups for those millions and millions of free accounts (for which they after all don’t make any such promises), and certainly not tape backups. I’m impressed.