Microsoft is also not writing all the drivers for Windows, this is the job of the hardware vendors.
Well you're half right. While actually writing the drivers is the business of the vendors, certifying that they work is Microsofts, or rather Windows Hardware Quality Labs' job (which they do quite poorly, I might add). Yes, it was always possible to run OSX on a PC, but Leopard does not do so out of the box, but apparently it's not all that difficult to do, although you do need a bit of luck, which I guess is to be expected anyway.
Oh but it gets better. Soon you'll be able to legally run the server version of Mac OS X Leopard on VMWare et. al.