Speaking of routing, I don't know for sure but I'd imagine most if not all of Google's services are on anycast IPs - meaning each node taking part in the DDoS w

Speaking of routing, I don't know for sure but I'd imagine most if not all of Google's services are on anycast IPs - meaning each node taking part in the DDoS would have its attack routed to its nearest Google DC rather than any central target. On a global scale this would mean that the one big DDoS you'd expect to see effectively gets split into many tiny DDoSes, which Google can handle using methods mentioned in the other responses in this thread.