Which further cements Zenimus' point, I think. When you have to do extraneous things to enable backwards compatibility, it is rightfully a different hardware platform. Mac OSX is especially selective when it comes to supporting old hardware and software, so I think it is fair to split it into three categories.
There isn't really an analog in Dos/Windows based computing, since Microsoft has been very diligent in trying to get older programs to run as well as possible on the newer OS. As far as I understand, since Windows XP, any 16-bit software has been run in an emulator of sorts. I for one wouldn't want to try to separate out my PC games to say: "This is a Windows 98 game, this is a Windows XP game" etc. Yuck.