Just had a thought while taking a shower the other day. These days, there's lots of effort to rate appliances by their efficiency. But it occurs to me that inefficiency leads to heat, and if you are heating your home, then you are basically using all that "wasted" energy. So even if some of the gas used for our water heater doesn't actually heat the water, as long as its in the basement and the heat travels upward, that heat is not going to waste. So the effective efficiency of the appliance is actually higher than expected. Conversely, in summer, if you use the air conditioner, the opposite is true. I guess the overall efficiency would depend on your mix of heating and cooling.
I was also thinking about this a while ago when I installed a bunch of LED lightbulbs. Although they use much less energy, they are producing much less heat to warm up the house. I mentioned this to Gautham, and he pointed out that using electricity to heat your house may be considerably less efficient than, say, natural gas, and so that means it's not 100% efficient, relatively speaking. Still, it's better than what one would naively expect.
Of course, the best thing about LED lightbulbs is not so much the electricity or cost savings (which are pretty modest, frankly), but the fact that they don't burn out. If you have a bunch of 50W halogen spotlights, you know what I mean. By the way, just got a "TorchStar UL-listed 110V 5W GU10 LED Bulb - 2700K Warm White LED Spotlight - 320 Lumen 36 Degree Beam Angle GU10 Base for Home" from Amazon, and it looks great (better than the other one I got from Amazon for sure).