How do I run a single Mac application on a Windows machine?

No, not really, unfortunately.

I mean, OK, there are some virtualized solutions, like http://pearpc.sourceforge.net/about.html, and I have never used PearPC, so I can't say first hand, but all signs are that PearPC sort of blows and really probably won't work for you. Still, can't hurt to give it a whirl I guess.

There is a VMWare OS X vmdk floating around on the nets that is dubious in legality (like hackintoshes) but works.

I think you can run OS X Server in virtual machine. At least, their license allows for it. I don't know if it's actually possible to do with existing products. If you were to get a Mac or do the hackintosh thing, things would be easier, since it's much easier to run Windows stuff alongside Mac stuff (VMWare Fusion has a kickass Unity mode where you can even have your Windows apps act like they're running alongside everything else). But I realize you probably don't want to switch.

Does this http://ask.metafilter.com/117990/still-a-pc help at all?

A used Mac Mini from Ebay/Craigslist and a cheap KVM would allow you to accomplish this for a non-trivial amount of money but not all that much, and you'd have a real Mac to boot. You certainly wouldn't need a recent model Mini to run this so you wouldn't have to spend too much. And yes, Scrivener is great and worth it!

If you really want to do this and money is no object, buy an Intel Macbook and dual-boot it in Windows and OSx.

@floam - You're allow to run OS X Server in a VM only on Apple hardware.

It is entirely possible to run http://wiki.osx86project.org/wiki/index.php/Vmware_how_to on windows. People... he doesn't want to buy a Mac to run a single application.