At work we have rack mounted power strips that can be controlled via serial or network connection, you can turn individual outlets on or off, turn the whole thing off or monitor amp load on the outlets. I don't think they were cheap, and are about 6 feet long, so not good for house, but shows the technology is there.
Now with Win 7, the Sleep/hibernate features work very very well, and most machines at idle take very little power. A PC that uses 120-200 watts during average use, can go down to 2-5 watts during sleep. At 2 watts it would take it 20 days to use a single kW. So probably not worth spending too much time/money unles you have many many high power machines. You could also look at wake on lan.
Edit: What I did buy for my house is one of those "smart" or "green" power strips (have a couple at home). It looks liek a normal power strip and is also a surge suppressor. But it has always on outlets, a master control outlet, and slave outlets triggered off the master. When my PC goes to sleep, my monitor, printer and otehr peripherals power off. Only my cable modem stays on the always on port since that takes 5 minutes to boot up and work. I also have one on my main TV that shuts power to the VCR, DVD player, game systems etc ewhen the TV powers off.