Where is Largest Linux Desktop Install?

jackb_guppy asks: "Talking with Managers about Linux Training for staff. One asked a simple question: 'Where is the largest installed base of Linux desktops?' My guess the question was asked prove that there is no market, and I am unable to find an answer. I am guessing the next question will be: 'Largest site using Linux to replace MS desktops?' Anyone have a suggestion?" Just for fun, if any of you have Linux Desktops deployed in your department, can you give us some numbers?