This is a calculation that depends highly on who you are and where you live.  I live in an area that recently had a 10%+ residental electric rate hike, to about 8 cents per KWH.  This is only slightly more expensive per btu than using natural gas with a 90% efficient gas heater versus a 100% efficent electric heater.  So the price difference for me to run any computer full tilt during the heating season, which is most certainly longer than Southern California, is about half a penny per kilowatt or less.  I don't even know anyone who bothers to shut down their computers from September to May to save money.  There's also someting to be said for the soothing white noise of a (good condition) cpu fan as the beast in the corner crunching numbers keeps your bedroom a couple degrees warmer so that you can turn the house thermostat down to 69 degrees at night.  I can't prove it, but I would bet that I actually save energy doing this, because otherwise my wife would insist on turning up the heat.