When MS first got Hotmail, it was all on linux/apache. They tried to put it on NT, and it failed miserably. I have no doubt they have linux/unix in their environment because any OS competitor would have multiple OSes/platforms to test stability, etc.

However, I also have no doubt that MS uses linux internally for things 'that can't go down". A lot of people are forced to use windows-anything because it's widely available and they don't have a choice. But, those same people often opt to use linux because of it's free pricetag and greater flexibility and stability. Believers are made from those given a choice.