I figured the tricky part would be automatic failover to a different data center when one goes offline. At least, I inferred that you had implemented that, from the part about you not having to get out of bed if a data center goes offline.
Ah, I get you. There's an HAProxy at the front of each data center "pod". If all servers within the pod go down, the proxy routes traffic to servers in another pod. If HAProxy itself goes down, a Route 53 health check just takes it out of the A Record rotation.