Lorentz Invariance, Not
Major Atmospheric Gamma-ray Imaging Cherenkov Telescope. … What MAGIC saw on that balmy June night … Lower-energy photons from Markarian 501 had outpaced their higher-energy counterparts, arriving up to 4 minutes earlier (Physics Letters B, vol 668, p 253) …
The MAGIC observations were showing just the sort of effect that quite a few models of quantum gravity predict. … A minimum size for space-time grains, as predicted by loop quantum gravity, could violate the cherished principle of special relativity known as Lorentz invariance, which states that the maximum speed of all particles, regardless of their energy, is the speed of light in a vacuum.
Here is more on the empirical issues; here is a solid and robust argument that a min size in space time implies a Lorentz violation. We seem to be starting to see a clear quantum gravity effect!
OK, uncle, I guess the result isn't as interesting as I'd thought.
Just to expand on what Steve Burrows wrote, I quote Lubos Motl in "fast comments" here:
'While the 2007 MAGIC paper may have speculated about 2.5 sigma signals suggesting "Yes, there is energy dependence", the new 2009 FERMI result is really a 100 sigma measurement saying "No, there can't be any". And believe me, 100 sigma is not 40 times statistically stronger than 2.5 sigma: it's exp(5000) times stronger.'
According to this null-hypothesis interpretation, the low-energy photons from Markarian 501 didn't "outpace their higher-energy counterparts", they were simply emitted earlier. And the difference in energies is due to the structure of the astrophysical source.