In the time it takes to develop a new game, I don't think XP will still be a big factor. Current video cards, etc, may all have XP drivers, but in 4 or 5 years that may not be the case.
Anyway, there is a difference between being capable of DirectX 11 and requiring it. As long as the current generation of the XBox is a significant factor in game sales, licensable game engines will have a big incentive to be able to fall back to DirectX 9 era hardware, to make development for the XBox and PC as seamless as possible and keep down development costs. Unigine, for example, supports DirectX 9/10/11, as well as OpenGL.
