One of the responsibilities - and joys - of doing science is in discovering something new, something that doesn't fit with existing theories. For that reason, it's important to continue to test those theories, and to test their limits.
NASA's Fermi Gamma Ray Space Telescope has been operating over the past year in this capacity. In 1905, Albert Einstein developed the theory of Special Relativity, which is based on the assumption that light travels the same speed under all conditions. This is hard to test thoroughly on Earth, because light travels so fast. The Fermi telescope, though, can record Gamma Ray Bursts (GRBs) - very short-term events (a few seconds at most) emitting a wide range of photon energies. GRBs sometimes occur very far away. The photons emitted race toward Earth for millions, even billions, of years, and we can record their arrival and calculate their relative speed.
In May, 2009, a GRB lasting 2.1 seconds detonated in a galaxy about 7.3 billion light years away. Fermi detected photons with energies about a factor of a million apart, which arrived only 0.9 seconds apart. Thus, the maximum difference in speed between the two would have resulted in a 3-second disparity in arrival time over 7.3 billion years (and possibly no disparity at all, depending on when the photons were actually emitted). This corresponds to an accuracy about 1 part in 100 million billion, or put another way, a difference of 3 nanometers/second in a speed of light of 300,000 km/second.
This is "good news" in the sense that it eliminates a class of theories about the structure of the universe. Some theories held that the speed of light actually was not quite constant under certain conditions, and would have predicted a much higher discrepancy that was measured here. Those theories must now be rejected or, at least, modified.
But it's sort of "bad news" (to the extent that any scientific fact can be given such value labels as "good" or "bad") in that it means we didn't discover a new discrepancy in our understanding of the universe. Such discrepancies are the source of all new theories; Einstein himself based Special Relativity on the Michelson-Morley experiment (which demonstrated that 19th-century theories about the propagation of light were fatally flawed). It would have been more "helpful" to find a large discrepancy in speed, which would enable us to reject all theories except those that admitted such a discrepancy. But, facts being facts, there's no use crying over it.
We know that the two great theories of 20th century physics, Relativity and Quantum Theory, are incompatible. The grand project of determining how they fit together just took another baby step.