What lighting or shading models would benefit from this, and looking forward, what functionality do you expect DX10 to allow that isn't currently handled by DX9?
VS 3.0 supposedly adds a texture read in the vertex shader. This should allow for hardware-accelerated programmable displacement mapping, for one. You could also use it to make the vertex shader "save" position of vertices it moves, "loading" the positions the next frame, for further CPU-offloading of dynamic systems.
A sampler register is an input register for a vertex shader, which is used to identify the sampling stage. There are four vertex shader sampling stage registers: s0 to s3. Four texture surfaces can be read in a single shader pass.