No matter how wonderful DX10 is, if the vast majority of gamers are still using XP then it's not a good idea to create a game that does not work on DX9.
Over the next few years, gamers will slowly switch over to Vista I have no doubt, but right now it's only a tiny minority.
If it's possible to create something that works on DX9 and DX10, obviously that's best - but the impression I got (And which may be wildly inaccurate) is that the two systems are not really that similar, and it would almost be like creating two separate games. I'm not sure that's feasible.
