Microsoft officially announced DirectX 11 at the Gamefest 2008 event in Redmond today. Just like DX10 the new API will be a Windows Vista (or higher) exclusive and it will introduce several new features like tessellation support, new compute shader technology for GPGPU computing and multi-threaded resource handling.
Full support (including all DX11 hardware features) on Windows Vista as well as future versions of Windows
Compatibility with DirectX 10 and 10.1 hardware, as well as support for new DirectX 11 hardware
New compute shader technology that lays the groundwork for the GPU to be used for more than just 3D graphics, so that developers can take advantage of the graphics card as a parallel processor
Multi-threaded resource handling that will allow games to better take advantage of multi-core machines
Support for tessellation, which blurs the line between super high quality pre-rendered scenes and scenes rendered in real-time, allowing game developers to refine models to be smoother and more attractive when seen up close
Microsoft also unveiled DirectX 11 will add new features to DirectX 10-compatible hardware but it's not clear what those features may be. A launch date wasn't provided either but it's expected that Microsoft will introduce DX11 next year.