The Most Distant Object?
The most likely power source of GRB's is the explosion of gigantic black holes. Size of a primordial Black Hole is limited by a "horizon distance" within light's reach. These gigantic Gamma Ray bursters exploding near the beginning of time is one indicator that the speed of light was once mych larger.
Further observations from Swift indicate that photons travel at the same speed regardless of their energy--a gamma ray photon travels as fast as visible light. This shows that c is not related to photon energy, but the speed of light may have changed with time.