Ok, I'm not sure how the business case is hard to make...
I run a London-based interactive design consultancy. We are 100% Mac based, as are the vast majority of our peers. It's hard to go anywhere in the industry without seeing a Mac.
This is not a Mac vs. PC debate - frankly, it doesn't matter what people chose to use. But the fact is, Windows is rare in the design industry (at least in the uk) and is even becoming extremely rare among hard core developers, due to the ability to virtualise multiple OS's on one machine concurrently (and the general excellence of tools like TextMate, OmniGraffle, etc. Oh, and the whole UNIX thing.).
We have a dozen machines we'd buy the software for if it ran natively, but we're not going to virtualise an expensive, high-grade app on a daily basis. So that's $6k lost, in just one small studio. Multiply that by hundreds (perhaps thousands) of similar studios.
Does that sound like a hard business case to make? Not to me.
If you don't do it, someone else will.