i restored to a previous save, and went through each option in the optimize scene size, and figured out it is the "delete unused render nodes" option, which is the same as the "delete unused nodes" in the hypershade.
now the easy thing would be to just go from the earlier save, and just not use that option. however, that's no way to learn. somewhere down the line, something like this is going to come up, and if i know why this is happening, i can avoid it and/or fix it.
using maya 7
rendering in mental ray, it comes up black, renders fine in maya software
doesn't seem to change any settings on the shader
only affects that one shader
the affected shader has transparency, reflection, and refraction, with ramps mapped to the first two to simulate the fresnel effect.
not using any 'bad' files in mental ray, just regular rgb/8 tiff files