I got confirmation of this from a nvidia rep on another forum yesterday (>= fermi cards should use WDDM 1.3), and I've since checked the support in the windows 8.1 preview with my 560Ti, using CheckFeatureSupport with D3D11_FEATURE_DATA_D3D11_OPTIONS1:
TiledResourcesTier D3D11_TILED_RESOURCES_TIER_1 (1) D3D11_TILED_RESOURCES_TIER MinMaxFiltering 0 int ClearViewAlsoSupportsDepthOnlyFormats 1 int MapOnDefaultBuffers 1 intThe updated Tiled Resources demo from page 2 of this thread is also working fine in hardware mode on this 560Ti.
So why was the card initially reporting as WDDM 1.2 in my posts on page 1 of this thread? Well it turns out my motherboard's bios default enables the onboard Intel HD 3000 display adapter in my CPU (Onboard VGA set to AUTO). Since no displays were connected, windows deactivates the device when windows loads, and it never appears in dxdiag or any device enumeration as a display device. I've never used it, never seen it shown as a display option, and never went poking in window's device manager looking for it. Even if you're not using it and windows doesn't present it as an available display device, it's driver is loaded and it's a WDDM 1.2 driver. By design or mistake, any further display devices are also made WDDM 1.2 devices (ie. pci-e graphics cards), even if their driver is WDDM 1.3 capable.
So in short, geforce 400 and 500 series also support the more useful directx 11.2 features, as well as the already confirmed 600 and 700 series. Just make sure any onboard VGA devices are explicitly disabled in your BIOS if you want to play around in the windows 8.1 preview with your nvidia card. As an aside, ivy bridge users shouldn't need to worry since the HD 4000 has a WDDM 1.3 driver included in the windows 8.1 preview.
Oh as a quick addition note: This probably also affects AMD cards used on sandy bridge systems with a HD 3000 capable CPU. You should explicitly disable onboard VGA in your motherboard bios, since the bios default may only 'hide' it in windows.