Old bug (2.42): when using node material, transparent shadow did not work. It was missing to set the proper 'pass flag'. Do note an important difference with non-node materials for 'transparent shadow'. If there are no nodes, it uses the color from the unshaded material. When it has nodes, it uses the color output from the entire node tree, which is typically from shaded materials. The latter is because node shaders have no support for shade passes yet (it only outputs rgb + a).