It seems Dx 10 will NOT be backwards compatible with respect to composite objects and UV mapping. This means Maya, Blender, etc. need new plugins to export to it as well. Which means more commercial Windows licenses sold to development houses. One of the given reasons is to make "Gaming more portable between the Xbox360 and PC's".
This is supposed to allow more game makers to release for the 360. The unspoken part is that it takes several hundred man-hours to make and uv wrap one character for a game (more for higher polygon counts). The only people with the development money already have the ability to deploy console games.