I have it stuck in my head that a photon is the way the EM field mediates energy exchanges. With this in mind, I think that a photon always travels at the speed of light in a vacuum, c, regardless of the material it is in. It's the EM field of the material that changes the speed of light in the material. It's actually the changes in the spin, momentum, and energy of charge and the associated absorbtion and re-emission of photons traveling at c that comes with these changes that gives an apparent change to the speed of the photon. I mean doesn't a photon in a vacuum always travel at c, so if a photon travels though a dispersive material in a vacuum, the photon entering is traveling at, the photon leaving will again be traveling at, and if there was a way to tell them a part they wouldn't be the same photon. Hang on is that it, the inability to tell photons a part is why we talk about the speed of light changing? Enough rambling.