Re: "It certainly won't be easier to administer"
For that matter, you can use Linux desktops in an AD domain, run a Linux machine as a domain controller, and interoperate both worlds. I understand the want to consolidate things and only support one family of OSes. From a user point of view, the experience probably won't be much different, other than I'd predict everything will run a bit slower and possibly less stable. Hardware requirements will go up too. From an admin point of view, there's going to be a learning curve whether you go from LInux to Win or vice-versa, and there will be some teething problems.
Since management usually only understands cost and not technical things (even some IT management), and Linux is a lot cheaper than Windows, Office, etc., it has to be that MS made them a really sweet deal. Probably something along the lines of "We won't charge you licensing for the first year, and you get free support from us, plus discounts on hardware from one of our partners." It's after that, that there will likely be hell to pay.