Oh, it probably will - it's just that while Nvidia keeps going in the fixed role shader units (pixel OR shader), Ati will keep churning out mixed role shader units (pixel OR vertex depending on case). Now, since DX10 was developed in close collaboration with Ati, it would stand to reason to think that Ati would make 'real' DX10 cards, while Nvidia won't.
Considering that in fact, vertex shaders can be emulated by the CPU at little cost, graphics cards may in fact include only pixel shaders, and not waste cycles on emptying a shader unit's pipeline, change its role, then refill its pipeline to do it all over again.
Frankly, the DX10 'revolution' is done more at the driver level than at the hardware level - DX10 cards won't be fundamentally different from DX9 cards, merely add a handful of functions to justify the upgrade.
Remember when Nvidia released the 'ti' series in the Geforce 2 range - and they turned out to be the exact same cards with a better driver? Well, it's pretty much the same thing here.
You may not expect your DX9 cards to run DX10 games with all the eye candy, but I'll be damned if there's no gifted demo maker able to make a demo run so similar on both DX9 and DX10 hardware no one would notice the difference.