nV News has kicked up an interview with Tony Tomasi and Ujesh Desai of NVIDIA on the recent launch of the GeForce 6800 series:

nV News:

Performance of the GeForce FX sometimes dropped when a Pixel Shader 1.x shader was executed compared to executing a similar shader under Pixel Shader 2.0. Will the GeForce 6800 have a similar performance drop when executing a shader using Pixel Shader 3.0?

Ujesh Desai:

Actually it will be the opposite. I think there is a bit of confusion about Shader Model 3.0 and Shader Model 2.0. While Shader Model 3.0 will enable some "new" effects, it is better characterized by ease of programming, more efficient use of the hardware, and higher scene complexity/or frame rates. Shader Model 3.0 makes developers lives easier due to the support for advanced programming features such as loops and branches.

This is a fundamental requirement and will improve the efficiency in how programmers can write their code. Without support for loops and branches enabled by Shader Model 3.0, developers will be forced to break up longer Shader Model 3.0 shader programs into smaller segments that will run on Shader Model 2.0 hardware. This will absorb clock cycles which will hamper performance in games that use the latest version of DirectX and have more sophisticated Shader Model 3.0 pixel shaders.

It is important to note that in some cases, developers can create the same effect with Shader Model 2.0 and Shader Model 3.0, however it may take longer to program using Shader Model 2.0 and may require more passes through the hardware to render.

Shader Model 3.0 does introduce some new functionality - particularly dynamic branching in the pixel shader, which must be used carefully for good performance. But in general, Shader Model 3.0 should actually make development easier, and can offer some nice performance benefits for complex shaders that can be executed in pixel shader 2.0, but can be executed more efficiently in Shader Model 3.0.