When looking at carbon emissions, Chicago is the greenest city for data centers, out of 21 cities researched, according to this study.
Personally, I am surprised that Chicago is even greener than Quincy, WA, the area attracting Microsoft and Google because of their low-cost hydroelectric power, and then significantly greener than other similar locations such as San Jose or Dallas. The study took into account the climate, lower temperatures mean more efficient cooling, the sources of the electricity, along with many other factors.
Here at Steadfast Networks, we had been looking into being more environmentally friendly, but where we are located there aren't many opportunities for such change, at least at reasonable costs. After seeing this study though I am much more comfortable, seeing that simply the fact that we're in Chicago is greatly reducing our carbon emissions. I'm not going to all of a sudden say we're a "Green Host," but according to that study you could definitely do much worse.
I don't care where you stand on the whole global warming "debate," but even if you don't believe man is harming the environment due to carbon dioxide emissions; it can't hurt that individuals and companies are working to reduce their carbon footprint.