The Financial Times has talked to several Google employees who confirmed the change in policy. The cited reason is security concerns; not entirely surprising after it became known several Google computers were hacked earlier this year. Google claims the Chinese were behind the attack, even though the Chinese government obviously denies this.
"We're not doing any more Windows. It is a security effort," one Google employee told The Financial Times. "Many people have been moved away from [Windows] PCs, mostly towards Mac OS, following the China hacking attacks," another employee said.
If you come to work at Google now, you're given the choice of either Mac OS X or Linux - in order to use Windows, you'll need CIO level clearance. "Linux is open source and we feel good about it," one employee said, "Microsoft we don't feel so good about."
Despite the validity of the security concerns, there's of course another reason this news has come out - and that's a marketing one. Google is working on the Linux-based Chrome OS, so promoting internal use of Google products is high on the agenda. "A lot of it is an effort to run things on Google product. They want to run things on Chrome," an employee said, "Before the security, there was a directive by the company to try to run things on Google products. It was a long time coming."