The big daddy of them all, the Vole itself doesn't care if your shaders are unified or not. You need to support Shader Model 4.0 to get the stamp of DirectX 10 and that's about it, but this doesn't necessarily mean your shaders have to be unified. This confirms NVIDIA's theory that you don't need unified shaders, at least for the time being. If the Vole doesn't care about it, why should NVIDIA? However, ATI has taken the unified shader approach for their upcoming new chip. The big green company decided to do the different approach as its vertex, pixel and geometry shaders will still be divided in at last two separate function parts and we believe that this will give some additional speed in DirectX 9.0 games. - The Inquirer

Microsoft DirectX 10 shaders need not be unified

That is how NVIDIA plans to make its G80 chip and Microsoft doesn't object. NVIDIA will be first to launch a DirectX 10 part with ATI to follow roughly two months after NVIDIA introduces its G80 baby. If that isn't quite a lead over the competition. And yes, it remains to be seen who will in the end will have the best accelerator for DirectX 10 games. But certainly NVIDIA will set the standard and reference, while ATI will just be a follower and will hopefully give NVIDIA a run for their money. It is just waiting now on NVIDIA to release their GeForce 8 series, with the GeForce 8800 GTX and GTS versions coming first.