Microsoft have released their Windows 8 Hardware Certification Requirements for devices.
The document is dated December 2011.
About the Microsoft Hardware Certification Program:
As always, the Windows Hardware Certification Program (previously known as the Windows Logo Program for Hardware) provides you the tools, guidance, and support to help ensure your product is reliable and compatible with Windows. For Windows 8, the Windows Hardware Certification Program will offer you a simplified process that is streamlined to focus on customer scenarios. If you create hardware products or drivers, we strongly encourage you to learn more about this program and its new requirements and policies.
Devices in this case I imagine include Windows 8 tablets.
You can download the Windows Hardware Certification Requirements for Devices here.