Hi, Jonathan,
Quark doesn't sell platform-specific licenses anymore, since QuarkXPress 7 they are platform independent. So you can even install and activate one license on both platforms, Mac and Win (you are just not allowed to use the same value simultaneously on two machines).
There should be no issues unless you use platform specifics. What comes to mind: Fonts and Graphics.
You can (and should!) try it yourself: The QuarkXPress 8 Test Drive is running for 60 days with full functionality.
Best
Matthias