"I'm a little confused at this, and I may be wrong, but in my tests IDL only lightens shadows by bouncing the diffuse component of ordinary lights."

I'm not arguing but I don't see this on my machine. (Intel I5, Nvidia Geforce)

Here is a night lit scene with IDL, SSS and GC. All objects and figures are light emitter enabled. PP 2012 with SR1.

Four lights: One diffuse (6%), one infinite (70%) for shadows and specular and two additional point lights for the lanterns.