Fat Knowledge has an interesting analysis on how many 'energy slaves' each American has working for them on a daily basis. An energy slave is defined to be some machine using the equivalent amount of calories that a typical American would use in a day (typically about 2200) The article will tell you how many, it's surprising (a three digit number).
I did my own conversion from 2200 calories, turns out it also equates to about 2.55 kilowatt-hours. That's rougly the energy to power a single PC for a single day. At that rate, I'm amazed that computers taken over the world yet; it certainly helps to explain their proliferation. At this rate Bill Joy is right, the future doesn't need us.
Electricity has long been the servant of the industrialized world. It has been cheap, abundant, and reliable. It's amazing the attiudes this cheap energy have created, and PC usage is one of them. It will be interesting to see what happens as the price of energy rises - will we keep the machines turned on, or keep the people employed? This is the sort of decision that has been made for decades in developing nations, but it will be a surprise for the developed ones.