It seems that if a consumer were to purchase new PC with Windows 8 and not like it, Microsoft are offering a downgrade to the OS to Windows 7 or Vista. Of course, there is small print, and this has also happened before, with Windows 7 Pro, which allowed a downgrade too.
The software license agreement for the version of Windows 8 Pro that will arrive as an OEM on new PCs says: "Instead of using the Windows 8 Pro software, you may use one of the following earlier versions: Windows 7 Professional or Windows Vista Business."
I think we're going to see more than a handful of people downgrading if the offer is there, it's also a nice sales tactic. If a customer was weary of the new touch-friendly Windows 8 Pro, they could always be offered a Windows 8 Pro-based PC that would allow for the downgrade, which would be a nice cushion to fall on if you didn't like Windows 8.
Windows XP isn't included in the list of operating systems that can be downgraded to, the downgrade only allows you to go back down the OS ladder as far as Vista.