[Bloody Sam Roberts]

"No such observational hiccups exist for the speed of light. "

Except maybe for a gravity lens. I know that a sufficient gravitational field can alter the path but can it alter the speed?
Isn't it a logical argument that if light can be slowed down by some outside force, then it can be sped up by some outside force? Just postulating.

[js1138]

I don't think anything slows light down. I have to refer you to physicist for a final answer, but when you say light propogates through some medium at a rate slower than he speed of light, I suspect it is bouncing around the molocules of the medium, traveling a greater distance.