There is a discussion going on in the Unity forums about a new shader system (link). Jason pitched a more modular approach for a Surface Shader 2.0 type system, and here is a proof of concept.
There are currently two examples, one of a bare minimum shader, which just returns green. The other is an example of a simple "snow on top" effect being applied to a regular shader. In this example, all of the data, code, and properties for the snow shader is contained in a single .subshader file.