The newest theory about Google's dark fiber purchases comes via eWeek, which reports that the goal of the mystery GoogleNet is to provide IPv6 connectivity, filling the gap left by existing providers' slow adoption of the next-generation Internet standard. The story quotes Alex Lightman, CEO of IP telephony vendor Innofone.com, as noting that Google has lots of network addresses configured for IPv6 and hired Internet protocol pioneer Vint Cerf specifically to oversee the project.
Microsoft's Don Dodge, noting eWeek's post, says Google's dark fiber buys were motivated by internal demand and the low cost of dark fiber - simply good business, rather than some overarching plan for world domination.
I asked Eric Schmidt about this dark fiber question a little over a year ago at the Nantucket Conference. His answer was fairly simple. Google has data centers all over the world with copies of the web index. They need the fiber bandwidth to keep the indexes in synch. Besides, it was available for pennies on the dollar from bankrupt telecom companies, so why not buy up a bunch.
Meanwhile, Google Operating System notes that Google video and its rapidly growing data center networks probably generate plenty of demand for bandwidth.