The other day I was asked "Why is cloud computing considered green?" Wouldn't you know, The Economist provided the perfect answer.
"In future the geography of the cloud is likely to get even more complex. “Virtualisation” technology already allows the software running on individual servers to be moved from one data centre to another, mainly for back-up reasons. One day soon, these “virtual machines” may migrate to wherever computing power is cheapest, or energy is greenest. Then computing will have become a true utility—and it will no longer be apt to talk of computing clouds, so much as of a computing atmosphere."