Velocity of light
If a region of space had a negative index of refraction, it would infer that "C" was locally > 3*10^8 MPS.
Materials with a negative refractive index have been fabricated. (see http://www.google.com/search?hl=en&ie=ISO-8859-1&q=%22negative+index+of+refraction%22&btnG=Google+Search )
Makes me wonder if just such a material could be used for FTL travel.