Windows 8 have RTM'ed and the availability are as following

August 15th: Developers will be able to download the final version of Windows 8 via your MSDN subscriptions.

August 15th: IT professionals testing Windows 8 in organizations will be able to access the final version of Windows 8 through your TechNet subscriptions.

August 16th: Customers with existing Microsoft Software Assurance for Windows will be able to download Windows 8 Enterprise edition through the Volume License Service Center (VLSC), allowing you to test, pilot and begin adopting Windows 8 Enterprise within your organization.

August 16th: Microsoft Partner Network members will have access to Windows 8.

August 20th: Microsoft Action Pack Providers (MAPS) receive access to Windows 8.

September 1st: Volume License customers without Software Assurance will be able to purchase Windows 8 through Microsoft Volume License Resellers.

Read all about it here