Hi Mike,
The shadow technique is for whole-scene shadow casting, including all objects. I have used it in the past, but not with glsl. It relies on building a depth buffer map taken from the perspective of the light source, then comparing it (via transform) with the depth buffer from the perspective of the viewer. If the depth values are less, then the pixels are in shadow, and toned accordingly.
Without further treatment, this produces hard-edged shadows.
But ambient occlusion looks very interesting, and offers more natural shading
http://http.developer.nvidia.com/GPUGems3/gpugems3_ch12.html
I am also looking at its (almost) opposite: radiosity, where scenes are shaded using ligh reflected from all the surfaces in the environment. Reflected light naturally takes on the colour of each surface it is radiated from. It produces very subtle shading, but quite a challeng to emulate in real-time!
