A Microsoft blog posting confirms: "The final version of Windows 10 will ship with DirectX 12". To me, this seems like a fairly obvious statement. The loose dates provided for both the OS and the availability of retail games suggest that the two would be launching at roughly the same time. The article also claims that DirectX 12 "Early Access" members will be able to develop with the Windows 10 Technical Preview. Apart from Unreal Engine 4 (for Epic Games subscribers), Intel will also provide source access to their Asteroids demo, shown at Siggraph 2014, to all accepted early access developers.
Our readers might find this information slightly disappointing as it could be interpreted that DirectX 12 would not be coming to Windows 7 (or even 8.x). While it does not look as hopeful as before, they never, at any point, explicitly say that it will not come to older operating systems. It still might.