Last year I met James Devine at homecamp a home hacking, automation and green technology unconference in London. He has been developing an arduino energy monitor based on an Analogue Devices ADE 7753 IC which is a custom energy meter IC for use with CT sensors. The ADE7753 looks like a really good way of doing the measurement, it communicates with the arduino via SPI and does all the energy metering calculations itself, therefore freeing up the arduino for other tasks.
Here's a mockup picture from his site of the energy metering arduino shield that he is developing:
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His project aim is to create an accurate desktop scale power meter, suitable for measuring individual power usage at a desk, the power meter then communicates with a PC I think via the arduino bluetooth shield and it's all open source, have a look: http://arduinopower.pbworks.com/