If you google to the Steam hardware survey where we poll peoples machines on Steam you can see that right now there is only about thirteen percent of those people that have both, the DX 10 card and Vista. So you have to look at that as well and say is it worth to investing for thirteen percent of the audience.
Oi!...i'm one of those 13%, i'm DX10.1 ready...i demand a game to use it!