While there is tons of speculation as to the real reason it is pretty clear at this point that Google real is encouraging people not to use Windows in favor of Linux and MacOSX. There are rumors all over the map as to the reason. The linuxinsider.com has a great rundown collection of the major rumors and a little bit of commentary. There are also articles over at zdnet.com the Googling Google Blog trys to claim that the switch will save Google millions, like they need the money and haven't already paid for the licenses.
This is truly great news for the Linux Community. Of course not the first major company to encourage the use of anything but Microsoft. Novell has the majority of it's company migrated and only those who can find no alternative software are allowed to stay with it. IBM and Cisco both allow people to choose their own PC. Basicly they give their staff a budget and provide solutions for them to implement and support. In the end this lowers costs and let's people work happier with the OS the picked. It is good to hear though and I hope more companies will start looking into why they have to stick with Windows. Especially for the IT Staff.