I understand. Now, if I have applications that have to run on Windows systems, and I go the route of doing it in virtualized Widows clients running under the Mac, will OS X manage those Windows machines? If not, what just happened to my TCO?
It would be part of your calculations. If all you have is that one Windows-only server app (or multiple apps that can go on the same server) among 20 servers you're going to come out ahead. As legacy servers get to be a large percentage of the total, with specialty apps that have no equivalent on the new platform, you have to think about not migrating.
It's all case-by-case, but if you want to stand up two networks for say 500 users, standing up an all-Mac system will cost you a lot less than an all-Windows system. The hardware may in cases be more expensive and you may need training for people used to Windows*, but saving on licensing and a few $40-$80K per year employees will save you a lot more than that.
* When Largo, Florida switched to Linux the training turned out to be easier than they thought it would be. Most questions were like "How do I change my wallpaper?"