This is pretty interesting New Scientist article about the Cloud:
"The cloud is often portrayed as greener than traditional computing, because any spare capacity in the cloud will always be less than what would be wasted if all users maintained their own dedicated servers instead.
In 2010 a study by Pike Research, a consultancy based in Boulder, Colorado, found that worldwide data centres forming the cloud could be using 38 per cent less energy by 2020 than a traditional computing infrastructure.
But the case for cloud computing being greener is not clear-cut. Rod Tucker at the University of Melbourne, Australia, recently applied supply-chain analysis to calculate the energy required to transmit and store data. He found that for some tasks, such as downloading many files, the cloud architecture uses more energy than a traditional setup."
The full article: The cloud's the limit