Yes, Geoff that's true in the pre-relativity physics. But the E=MC^2 argument is there in all physical and chemical reactions, not just when an electron (matter) and a positron (matter) go bang and dissappear to produce a photon of energy. If you heat up anything, its mass will increase according to the famous equation but the ammount is so small that you couldn't measure it. Anything actually moving will have a slighly increased mass because of its kinectic energy; this mass returns to the 'rest mass' once it stops. To take this further at the speed of light a body has infinite mass so cannot accelerate further: you'd need an infinite force to do this. This is why those particles at CERN travel at nearly the speed of light but not at it.