The Matrix script was originally that humans were used for our processing power (instead of the dumb idea that we somehow generate energy without input that ultimately comes from the sun).

Besides all the other good points in this thread (concerning open-source software, etc), there's a good probability that the nature of work will change, as human brains are extremely good at performing certain tasks with far less energy input than computers. One possible scenario is that we are utilized as a distributed super-computer. It will be interesting to see what types of human-machine interfaces will be utilized.