Do you believe that? I always figured it was Microsoft's lame excuse to get people to move quicker from XP to Vista. I guess we'll see a few months after DX10 is released; some hacker will probably try to port the entire API to XP.. and succeed.
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Having thought about it overnight, I can see that it isn't impossible ... but it certainly wouldn't be trivial, either. Who knows how many and deep the DirectX graphics subsystem's roots are embedded ..?
Also, nVidia has (allegedly: it's all rumour at this point) chosen a conservative implementation for its G80 (DX10) chip: 32 pixel shaders and 16 unit array for vertex and geometric work (as opposed to ATi's R600 which apparently has four unified 16 unit arrays (64) up from the current R580's 48 pixel and 8 vertex shaders). The reason this is interesting is that the nVidia chip isn't strictly a unified design, so it sounds like it might be possible to break down the Windows Display Driver Model (WDDM) instructions into something that a DirectX9 GPU might understand.
On the negative side, however, is the main benefit for DirectX10 is to reduce the admin overhead for drawing more objects on screen, as well as making a madatory code path for drivers (so no more engines optimised for one GPU over another).
So, on balance, I don't think it would be worth the time, money and effort to port DirectX 10 to XP, as it sounds like it would be too slow to be useful.
But there are a lot of ifs and buts in there.