WDDM 1.2 remains a Windows 8 exclusive though, including the support for stereoscopic 3D without additional software.
Here is a quick summary of what KB 2670838 adds to Windows 7 Service Pack 1 / Windows Server 2008 R2 Service Pack 1:
ID3D11Device1, ID2D1Factory1, IDWriteFactory1, IDXGIFactory2, IWICImagingFactory2, ID3DUserDefinedAnnotation and related APIs are available. Methods that depend on WDDM 1.2 drivers or new Windows Store app APIs are not supported.
Improved Direct3D 11 device interoperability via ID3DDeviceContextState, including the improved interop with Direct2D/DirectWrite
D3D11_FEATURE_DATA_D3D9_OPTIONS feature detection
In addition to the new Windows 8 WIC features, this update also fixes decoding of 96bpp floating-point TIFF images.
All other features of DirectX 11.1 are Windows 8 / Windows Server 2012 only.