Give it to me straight: How does the Windows 8 licensing actually work?

The official word on Windows 8 licensing is that you need to pay $40 per machine to upgrade from Windows 7 to Windows 8. If you buy 1 license and try to use it on a second machine, what actually happens? Does it say the serial has already been used, or does it work fine? Adobe software is serialized and is 1 install per license, but you can install it on a hundred machines if you wanted to, you would just be violating the EULA. So, what's the real deal? Someone, anyone?

Thanks.