The current licence agreement for OS X actually specifies "Apple-branded" rather than "Apple-labeled" computer. I believe this change was made to clear up the ambiguity of exactly what was meant. After all, Apple supplied a couple of labels with each retail copy of Leopard, so stick one of those official Apple labels on a PC and it's an Apple-labeled computer? But the current licence is quite clear - you can only install OS X on a genuine Apple computer.
In reality, you can't legally obtain a copy of recent versions of OS X unless you already own a Mac. So those copies that can be downloaded off the 'net are all pirated. And we don't discuss pirated software here.