Quote from CNN: They get quotes from a number of employees who say the reasoning behind the move is security-related and specifically a result of the recent Chinese hacking incident...
"We're not doing any more Windows. It is a security effort" "Many people have been moved away from [Windows] PCs, mostly towards Mac OS, following the China hacking attacks" "Particularly since the China scare, a lot of people here are using Macs for security" The recent Chinese hacking of Google was done through Microsoft (MSFT) Windows computers on the Google's (GOOG) corporate network.
Windows is technically still an option, but you need to have some seriously good reasons to have it on your desktop. Those reasons need to be justified to the office of Google's CIO and approved. Otherwise, it is a Mac or a Linux box.