DirectX 10...and so it ends!
There are troubled times ahead. Through one bold move, Microsoft has decided that it will not support, the already too “old” DirectX 9, not to mention DirectX 8 or any of the previous versions. But... it appears to be some logic hidden somewhere.
The new DirectX will not be named Windows Graphic Foundation (WGF) as it was planned and will remain at the old name, as in DirectX 10. It will be released with their brand new operating system Vista. This news API will be composed of new and faster dynamic link libraries (DLLs) and will run much faster (so they say).
Microsoft has decided that backward compatibly with DirectX 9,8,7 isn't really necessary as there will probably will be even less compatible with Vista.
Even so, dear Microsoft hasn't totally forgotten us. Some sort of “compatibility” will be available through a software layer (probably some emulation) which will have its price in system resources, as it will run much slower. The good news is that DirectX 10 will relieve some of the burden on the CPU.
And of course it will have support for the next generation of Pixel Shaders 4.0, although it will probably surface before even Vista's release due to the rapid development of graphic cards.
http://news.softpedia.com/news/DirectX-10-and-so-it-ends-7762.shtml