That's exactly the reason why we're not seeing many DX10 games--the vast majority of computer users still use XP.
The vast majority running XP don't have computers capable of running most recent AAA title games. And if you for some reason have a Core i7 / GTX 295 rig and are running XP, allow me to laugh maniacally at you for wasting your hardware.
It's simple logic, though - why even bother with DX10/11 if you are trying to keep dev costs down when your current-gen game consoles are only running halfway-decent DX9-class hardware?