"Direct3D 11 runtime introduces Direct3D 9, 10, and 10.1 "feature levels", compatibility modes which only allow the use of hardware features defined in the specified version of Direct3D. For Direct3D 9 hardware, there are three different feature levels, grouped by common capabilities of "low", "med" and "high-end" video cards; the runtime directly uses Direct3D 9 DDI provided in all WDDM drivers."
Oh well, We will see when some dx11 games come out.
1. it states DX10 can run on DX11 hardware. Read it below
"The next version of DirectX (DX11) will fully support Windows Vista (DX10) as well as upcoming versions of Microsoft’s operating system (Windows 7), as well as all graphics cards designed for DirectX 10 and 10.1, the company revealed during its GamesFest event in Redmond."
so new DX11 card will support (aka be able to run) DX 10 games as well as DX11 games.
2. DX11 games are out, you cant run DX11 on DX10 cards, its not even an option in the menu.