I guess I also wonder if heating a house (partially) on electricity is "greener" than 100% natural gas?
That's an interesting point ... our house is designed to allow us to heat a thick concrete slab over night and (in theory) not run the heating as much during the day. The local utility companies are on the brink of introducing time-of-day hydro rates, so we will pay less for a BTU of heat generated via electricity off-peak than we would for one coming from gas.
As to it being greener ... whilst electricity generation itself is way less efficient than modern gas furnaces, what happens to all the electricity that is generated overnight that isn't consumed? That is getting beyond my understanding of how the power generation and distribution systems work.