Do you remember how bad reflections impacted performance in Lock On? Imagine if instead of using a simple image of the cockpit to simulate reflections, it also had to simulate refractions/diffractions through moving water on the glass. Maybe I'm wrong here as I'm not exactly a programmer, but it seems to me that it would be far more resource heavy than the current tech could handle.
Regardless of whether or not they make use of DX10, I highly doubt XP users would be left in the dark. I've never seen a game that was only compatible with DX10. They might exist, but no independent developer serious about their product would do that: there's no reason for it other than increasing MS's revenue. And despite all these complaints about Vista's memory handling, I get consistently better fps than in XP.
Before the hounds of XP hell are unleashed upon me, let me say that this is not directed at any of the above posters and I have absolutely no interest in starting another bible-sized OS debate; I'm just getting tired of all the XP 4 LIFE!!! dudes. Sure, it's a great OS, but it wasn't released like that. Vista had a rockier start than XP, but since I've installed it I've had very little to complain about, least of all performance. But that's just my very narrow experience and it might be the worse thing ever on every other computer than the ones I've used, however I somehow doubt that's the case. It seems to me that most of people's reluctance to switch over comes from either stubbornness, impatience or money. I completely sympathize with the latter, but to the others I say grow up.