Originally posted by Gh0stFT
Danish physicist Lene Hau, brought light waves down to a 1 mph crawl by putting
them through a specially prepared haze of ultracold sodium atoms.
Some of the physicists accomplished what sounds like an impossible task:
slowing down a light pulse so much that it appears to fade and stop, then starting
it up again on demand.
looks like an easy task ! 
Again, that is not really a measure of the speed of light "
c", it is a measure of the refractive indexes of the substances the light went through. What you are talking about is NOT the "speed of light", the constant,
c, it is the speed of light in a medium other than a vacuum. Those are different animals. Sort of. In other words,
c is a constant, the speed of
c in air is slightly less because of outside forces acting on it. Here, this explains it better.
Interaction with transparent materials
The refractive index of a material indicates how much slower the speed of light is in that medium than in a vacuum. The slower speed of light in materials can cause refraction, as demonstrated by this prism (in the case of a prism splitting white light into a spectrum of colours, the refraction is known as dispersion).In passing through materials, light is slowed to less than c by the ratio called the refractive index of the material. The speed of light in air is only slightly less than c. Denser media, such as water and glass, can slow light much more, to fractions such as 3/4 and 2/3 of c. This reduction in speed is also responsible for bending of light at an interface between two materials with different indices, a phenomenon known as refraction.
Since the speed of light in a material depends on the refractive index, and the refractive index depends on the frequency of the light, light at different frequencies travels at different speeds through the same material. This can cause distortion of electromagnetic waves that consist of multiple frequencies, called dispersion.
Note that the speed of light referred to is the observed or measured speed in some medium and not the true speed of light (as observed in vaccuum). On the microscopic scale, considering electromagnetic radiation to be like a particle, refraction is caused by continual absorption and re-emission of the photons that compose the light by the atoms or molecules through which it is passing. In some sense, the light itself travels only through the vacuum existing between these atoms, and is impeded by the atoms. The process of absorption and re-emission itself takes time thereby creating the impression that the light itself has undergone delay (i.e. loss of speed) between entry and exit from the medium in question. It may be noted, that once the light has emerged from the medium it changes back to it's original speed and this is without gaining any energy. This can mean only one thing - that the light's speed itself was never altered in the first place. Alternatively, considering electromagnetic radiation to be like a wave, the charges of each atom (primarily the electrons) interfere with the electric and magnetic fields of the radiation, slowing its progress.
