According to the compute engine talks from this year's google IO, they have their own private network covering most of the world and uses anycast for external Internet Addresses. This basically means that a connection from you will be routed to the nearest google data center, and from there to the final server using google's private network.

Google DNS uses both anycast and multihoming, so you will both be routed to the nearest google data center and then to the closest DNS server inside google's private network.