I dont know about the XP/Vista thing, interestingly DX10 will only work with DX9,8 etc through a software layer, though the impact of that on performance remains to be seen.
AFAIK DX10 hasnt been totally defined by MS, i have read SM4.0 "and beyond", so i presume the cards will come along with the definition as both ATI and Nvidia will rush to have the first DX10 cards.
When they will release the definition is probably up in the air again now given the release of Vista has been pushed back.
No answers really in this post i know, but thought i would muse a bit, i could be totally wrong of course