From http://216.239.57.104/search?q=cache:kVfI1Wb2qmIJ:www.computer.org/micro/mi2...

"To provide sufficient capacity to handle query traffic, our service consists of multiple clusters distributed worldwide. Each cluster has around a few thousand machines, and the geographically distributed setup protects us against catastrophic data center failures (like those arising from earthquakes and large-scale power failures). A DNS-based load-balancing system selects a cluster by accounting for the user’s geographic proximity to each physical cluster. The load-balancing system minimizes round-trip time for the user’s request, while also considering the available capacity at the various clusters."

Sounds like they're running a sort of Super Sparrow as well...

Some strange pics: http://backrub.nerisoft.com/May1998/hardware.htm