AFAIK, the requirement is due to security concerns from the China hacks in 2010. The way Sergey Brin put it in 2011, it sounds like employees have the option. On the number of employees using Windows: "I’d probably guess 20 percent ... But I’d have to get back to you."

http://www.ft.com/intl/cms/s/2/d2f3f04e-6ccf-11df-91c8-00144... http://techcrunch.com/2011/05/11/googlers-windows/