- Upgrade -
Given that one may officially only install the product on "an Apple-labeled computer", and every Mac comes preinstalled with OS X, a case could be made that the boxed version is really an upgrade. This is however hard to defend as the AppleTV is an Apple-labeled computer and runs a version of OS X, but it would be illegal to run Mac OS X on an AppleTV.
- OEM -
When you buy a boxed Mac OS X, you can only install it on Macs that meet the requirements. Support through Apple is minimal, you're expected to have bought AppleCare for your computer, which then gets you OS support as well. This could pass for some form of OEM deal.
- Retail -
Retail, ie. "buy the product, and do with it as you please" seems like the least plausible option. There is only one Mac OS X Leopard Client, and for most customers it costs $129. This is way less from what Microsoft charges for their full featured 64-bits Vista retail version. Has Apple ever been known for their bargains?
I think Psystar might be mistaking the boxed version for a retail version. In the unlikely case that the win the case however, I can see Apple creating a full retail version and an upgrade version, with the full retail version being > $1000, so that it won't make any sense for Psystar to sell the systems anymore.