Excellent idea, i've not used compost to heat water, but rather to heat waste veg. oil for biodiesel making. Very low tech approach, as i just buried 5gallon jugs into the pile and let them sit overnight. Heated the oil to about 97degrees, with an ambient air temp of about 40 or so. So its definitely do-able.
I remember an article in an old Mother Earth News that did exactly what you propose. I recall they ran black plastic pipe in a coil under a compost pile, and were able to acheive a significant temp. rise for a few months.
best of luck to you.