Why is this big? Because the speed of light in a vacuum is considered to be the closest thing to an "absolute" there is in the world of modern science. It is the touchstone upon which most of the empirical substrates of modern science. Every single theory of what we "know" goes in the trash can and we look for an entirely new model for subatomic physics if this turns out to be true.
BBC News - Speed-of-light results under scrutiny at Cern