NVIDIA has posted a useful whitepaper detailing the ability to access texture information from vertex shaders, which was introduced in Shader Model 3.0 on the GeForce 6 hardware.
"Since the introduction of programmability to the GPU, the capabilities of the vertex and pixel processors have been different. Now, with Shader Model 3.0, GeForce 6 Series GPUs have taken a huge step towards providing common functionality for both vertex and pixel shaders."
"Using Vertex Textures whitepaper focuses specifically on one such Shader Model 3.0 feature: vertex texture fetch. Vertex texture fetch allows vertex shaders to read data from textures, just like pixel shaders can. This additional feature is useful for a number of effects, including displacement mapping, fluid and water simulation, explosions, and more."
More information can be found here