I've found some references, when looking for information on WAIK installs of Windows 8 of a varient called Windows 8 core, that can be installed off the professional edition disk. There's also a mention of it on Win Super Site that says
The cheapest, called MSDN Operating Systems, costs $699 a year ($499 renewal), and of course includes Windows 8 (Core), Windows 8 Pro, and Windows 8 Enterprise. You get five licenses each for Windows 8 Core and Pro, and one Multiple Activation Key (MAK) for Windows 8 Enterprise.
What is Windows 8 core? Is it the new 'home' edition or something else? What differentiates it from the pro and enterprise editions?