Here is an article that argues that it is really wireless & 4G networks that energy-concious people should be concerned about, not bitcoin or datacenters: http://www.washingtonpost.com/blogs/wonkblog/wp/2013/04/12/what-bitcoin-teaches-us-about-the-internets-energy-use/ (and the corresponding post I just made on the bitcoin forums: https://bitcointalk.org/index.php?topic=175429