No word on whether it's an exclusive. http://www.bit-tech.net/news/bits/2014/10/03/ms-dx12-windows-10/1
It will very likely be a Windows 10 only. Reasons being that 11.1 is already Windows 8 only and Windows 10 is going to be a free upgrade for Windows 8/8.1 anyway there is no reason to support anything else unless they have a change of heart and support 7 (which they wont)
A few dx12 tech demos: http://channel9.msdn.com/Blogs/DirectX-Developer-Blog/DirectX-Techdemo https://www.youtube.com/watch?v=W1lHeLN1UGs https://www.youtube.com/watch?v=CJmXWQnDSqg