Windows domain group policy doesn’t always work on *Windows* machines, for that matter. It certainly doesn’t work on Linux boxes.
And there *are* tools for centralized management of Mac OS X machines that do scale. However, they aren’t able to manage Windows machines any better than AD can manage a Mac. Likewise with Linux.
All it means is that your IT personnel have to actually earn those salaries. :D
It also means that it’ll take more staff and infrastructure on the back end to support it. Now the question is, how much is that going to cost, and what are you really getting out of it?