Someone on the bitcoin forums[1] has a killawatt meter and measured the difference between idle and 100% CPU. We did the math and for his computer, if the cost of electricity is under 14.1 cents per kWh (and you have your computer on anyway), it's profitable to use my CPU miner.

[1] http://forum.bitcoin.org/index.php?topic=8780.msg127782#msg1...