Well since I have not actually gotten my hands on a DX10 card yet, nor are they even out for consumers anyways....I can only assume that DX10 cards will be backwards compatible with DX9, or MS will release DX10 for XP.
DX10 is suppose to fix a lot of the overhead that DX9 had, and of course intergrate new technologies as well. Now, if the first is true then manufacturers will want legacy support for XP machines so they will probably release drivers, however since MS says no DX10 on XP you wouldn't benefit from the features I suppose. However, that would bring up an even bigger problem for developers, which means they might have to bump up their minimum requirements on their games to Vista compatible OS. Which is another way to force you to upgrade to Vista, which IMHO is crap.
Anyways, we won't know anything until DX10 hardware becomes readily available. I can only imagine that they will end up supporting XP systems. That makes sense if they want to keep their customer's and developers happy.