DirectX 10 will be released with the Windows Vista.
XNA has not been developed to replace anything.
What exactly is the XNA Framework?
The XNA Framework allows game developers to create modern games using the C# programming language and a rich set of development libraries. The XNA Framework provides its own content pipeline to make it easy to get rich content (3D, 2D, sound, etc.) from content creation sources into a game. The XNA Framework also provides a very high level of API unification across the Windows and 360 implementations, thus virtually eliminating the cost and effort required to port games between these two platforms.