There was an article on the Inquirer about that i think … Found the link here it is: http://www.theinquirer.net/default.aspx?article=40913

I think MS was talking mostly about the new display driver model(WDDM) as the main reason dx 10 will be for vista only.

Some more links:
http://msdn2.microsoft.com/en-us/library/bb219721.aspx#Will_DirectX_10_be_available_for_Windows_XP
http://windowsvistablog.com/blogs/windowsvista/articles/447226.aspx

http://alkyproject.blogspot.com/ Interested to see how this turns out . ;D

And here you can read how todays dx10 card do in dx10 games . Not very promising.
http://www.anandtech.com/video/showdoc.aspx?i=3029