Thanks - I'll read up. I've long interpreted E=mc^2 to mean not just that energy and mass are interchangeable, that energy and mass are one and the same.
What I can't quite get my head around yet (just need to read up more) is how they figured out something about the rest mass of a photon - something about using superconducting rings or something like that.
(Because photons aren't generally at rest, and they don't accelerate, right?)
Fun fact: One of the ways physicists are looking for Higgs is by looking for bumps in the "invariant mass" spectrum of LHC events with 2 photons!