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
Matthias Günther
Senior Product Manager
Quark Software Inc.(Please note: As I am traveling frequently, answering your post might sometimes take longer)
Want to easily publish for the iPad, using high-fidelity designs with stunning interactivity? See here: English:
http://youtu.be/Gldk5lvXXTA French:
http://youtu.be/nsgB4Q7lQzg German:
http://youtu.be/lecdenqaUGY Italian:
http://youtu.be/wjBpYZsF-8s
Need help? Contact Quark's support:
http://support.quark.com/contact_us.html
Please don't install the newest update 10.7.3 of Mac OS X Lion yet. For details please see here: http://goo.gl/AzKeR