If you haven't gotten the message, Microsoft really, really wants the 11-year-old Windows XP dead and buried. The company previously said the forthcoming Office 2013 won't run on XP. Now it says buyers of Windows 8 Pro won't be able to legally apply that license to Windows XP, so they can't put Windows XP on a PC shipping with Windows 8 Pro. (You need to supply your own copy of the previous version of Windows, by the way.)
Microsoft's revelation that it won't let buyers of Windows 8 Pro replace Windows 8 with Windows XP on their PCs shows one more nail being pounded into XP's coffin. Windows 7 and Vista "downgrades" are allowed. Windows 8 Enterprise has similar downgrade rights that exclude XP. The Windows 8 basic edition has no downgrade rights at all -- similar to how Microsoft handled downgrade rights in Windows 7.
There's little practical effect for most users in Microsoft's no-XP decision. Microsoft will let PC makers sell PCs with Windows 7 at least through October 2014, and Windows 7 Professional includes downgrade rights to XP. A company that has not migrated to Windows 7 (most still haven't) can buy new Windows 7 PCs after Windows 8's release, then install Windows XP on them to remain compatible with their existing environment.
By late 2014, most businesses are likely to have upgraded to Windows 7 -- most are now in at least the planning process to do so -- and when they can no longer buy Windows 7 licenses then, they'll still be able to downgrade the Windows 8 PCs of that era to Windows 7. One hiccup could be driver compatibility between new PCs and the old operating system, but that's always a downgrade risk, and business PC makers like Hewlett-Packard and Dell typically factor that issue into their designs.
Microsoft's removal of XP from Windows 8 Pro's downgrade rights is mainly symbolic -- just a little reminder in case you haven't heard the other nails being pounded.