Unity3D shader that will make an object transparent, but still cast shadows.

InvisibleShadowCaster.shader

Shader "Transparent/InvisibleShadowCaster"
{
    Subshader
    {
       UsePass "VertexLit/SHADOWCOLLECTOR"    
       UsePass "VertexLit/SHADOWCASTER"
    }
 
    Fallback off
}