Is that c the speed of light in vacuum or c the constant in special relativity?
c is the constant as it appears in fundamental physical equations, relativistic or quantum. Anything slowing down the propagation of photons through an apparent vacuum (such as interaction with dark matter) which did not affect, for example, the mass-energy equivalence of E=MC2, would not win the bet.
http://www.nature.com/news/2011/110922/full/news.2011.554.html
http://arxiv.org/abs/1109.4897v1
http://usersguidetotheuniverse.com/?p=2169
http://news.ycombinator.com/item?id=3027056
Perhaps the end of the era of the light cone and beginning of the era of the neutrino cone? I'd be curious to see your probability estimates for whether this theory pans out. Or other crackpot hypotheses to explain the results.