> Aren't shaders used mainly for pseudo-realistic effects (such as creating
> shadows by superimposing a transparent texture generated from a stencil 
> buffer)
> rather than proper raytracing?

Yes, but that's only because they are mainly used by GPUs for real-time 
applications.  No reason at all why something similar can't be used in POV.

> How do you envision a shader being merged with a
> full-fledged raytracer such as povray?

What the shader must do, is to take some data as input (eg world space 
coordinates, uvw texture coordinates, surface normal etc) and return a 
colour value.  You could just define it as another user defined function 
type in POV.

Maybe something like:

#shader myShader( worldPos , texCoords , normal , rayDir)

 #local color = <0,0,0,0,0>;

 // add some suface normal modifier
 #local normal += noise3d(worldPos)*0.1;

 // add some basic ambient
 #local color.rgb += <0.1,0.1,0.1>;

 // loop through each light in the scene
 #local n = 0;
 #while( n < nLightsInScene )

  // add simple diffuse lighting
  #local color.rgb += vdot(normal,light[n].direction) * <0.7,0.4,0.4>;

  // and some specular
  #local color.rgb += pow( vdot(vreflect(rayDir,normal),lightDir[n]),40)) * 
<1,1,1>;

 #local n--;
 #end

#end

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