Concrete is a good conductor of heat, and therefore not a great insulator, which is why lots of 1960's concrete blocks of flats have such terrible condensation problems when the occupiers want to have their homes warmer than outside.

Have you checked the hockerton housing association (in the links tab above under interesting projects), they have effectively built using concrete as the structural element which then acts as the main thermal store. But they have insulated around this using I think 300mm of polystyrene, to prevent the heat stored in the concrete being lost to the environment. They havealso banked over the concrete and insulation with soil, so from the north side the terrace looks like a flowing bank of grass.

If concrete is used, with its high embodied energy, this is the only real way I think it can be justified, but certainly not in the indescrimated thoughtless way it is usually used on many building sites, especially when there are alternative solutions available.