Windows 8.1 and Server 2012 R2
Windows 8 Home users can upgrade here
MSDN (If you have a subscription) Has the Server 2012 R2 ISO now and should have Windows 8.1 sometime after 9am on June 26, 2013. https://msdn.microsoft.com/en-us/subscriptions/securedownloads
The Windows Enterprise Website The Windows 8.1 Launch page http://www.microsoft.com/en-us/windows/enterprise/products-and-technologies/windows-8-1/default.aspx
Enterprise ISO Download Link http://windows.microsoft.com/en-us/windows-8/preview-iso
Make sure you watch the Keynote from Build 2013! http://channel9.msdn.com/Events/Build/2013
Should you want to give Microsoft feedback on your testing, you can sign up at Connect, but I don't think they are taking 8.1 testers right now.