Originally it was touted by MS that the only OS that would be able to run DX10 would be Vista because of a few things, I think virtualisation was the main deal breaker.
However, the graphics manufacturers have had a problem getting their drivers, etc. to work, ATI were able to do it and Nvidia weren't so that requirement was quietly dropped I believe. So it may be possible to keep XP and eventually run DX10 games.
As Mac said it will take time before the Games Manufactures feel there is a sufficient user base to make it worth their while, if XP can run DX10 then that would be a greater reason to push forward. By that stage M$ will be about ready to announce the replacement for Vista

Excuse taking this topic off course.