The Wikipedia article "Mass–energy equivalence" is very useful http://en.wikipedia.org/wiki/Mass%E2...gy_equivalence and I can understand this very well. It only seems to breakdown when describing Gravitational Potential Energy. They have not worked out that it contributes to the Rest Mass of an object.
Quoting from the article:
"Note further that in accordance with Einstein's Strong Equivalence Principle (SEP), all forms of mass and energy produce a gravitational field in the same way.[15] So all radiated and transmitted energy retains its mass. Not only does the matter comprising Earth create gravity, but the gravitational field itself has mass, and that mass contributes to the field too. This effect is accounted for in ultra-precise laser ranging to the Moon as the Earth orbits the Sun when testing Einstein's general theory of relativity.[15]
According to E=mc2, no closed system (any system treated and observed as a whole) ever loses mass, even when rest mass is converted to energy. All types of energy contribute to mass, including potential energies. In relativity, interaction potentials are always due to local fields, not to direct non-local interactions, because signals cannot travel faster than light. The field energy is stored in field gradients or, in some cases (for massive fields), where the field has a nonzero value. The mass associated with the potential energy is the mass–energy of the field energy. The mass associated with field energy can be detected, in principle, by gravitational experiments, by checking how the field attracts other objects gravitationally.[16]
The energy in the gravitational field itself has some differences from other energies. There are several consistent ways to define the location of the energy in a gravitational field, all of which agree on the total energy when space is mostly flat and empty. But because the gravitational field can be made to vanish locally at any point by choosing a free-falling frame, the precise location of the energy becomes dependent on the observer's frame of reference, and thus has no exact location, even though it exists somewhere for any given observer. In the limit for low field strengths, this gravitational field energy is the familiar Newtonian gravitational potential energy."
When they start talking like this "the precise location of the energy becomes dependent on the observer's frame of reference, and thus has no exact location, even though it exists somewhere for any given observer" I would suggest you can tell they have got it somewhat wrong!
I will take a bet it will be shown one day that GPE is stored in the mass of the objects, even though I do understand there is a Gravitational Field around these masses which means that some of the Mass-energy is tied up in the production of this field. The fact that it is energy related would tend ultimately to allow for proposition of a quantum of Gravitational Field Energy, which then suggests it would only have a finite effect and not extend out infinitely into space.