I don't know whether this guy knows what he's talking about, but it sounds plausible:

Steven Sudit:

The speed of light in a typical vacuum false short of the speed in a perfect vacuum. Light is slowed by interaction with particles, even the virtual particles found in a vacuum. This is why it's slightly faster when passing between plates exhibiting the Casimir effect, since that's based on suppression of virtual particle creation. (http://en.wikipedia.org/wiki/Faster-than-light#Faster_light_.28Casimir_vacuum_and_quantum_tunnelling.29) So one plausible explanation is that, because of their minimal interaction, neutrinos travel at the speed of a true vacuum, slightly edging out photons.