Despite some recent studies that indicate the speed of light may have been slowing down, NASA has announced results of examination of light from two nearby galaxies showing that the speed of light is as special relativity describes it.
Specifically, they looked at Mkn 421 and Mkn 501, both of which have massive black holes spewing gamma radiation. The gamma photons collide with infrared photons and annihilate each other. If the speed of light were not constant, the photons from the galaxies would not have enough energy to cancel out the infrared ones.
There has been some investigation into whether or not the quantum foam would impeded the speed of light. It also brings into question some models of a theory of everything involving extra dimensions. The results indicate that the energy levels of these dimensions can't be as powerful as some of the models claim them to be.