We had to pull together a lot of disparate data sets to generate some answers.

Location, location, location

According to the IEA data on CO2 emissions per kWh from electricity and heat generation, Iceland and Norway are the only developed countries averaging <10 grams Co2e/kWh. (Some developing countries also have very low CO2e/KWh, but other infrastructure issues make them unlikely candidates for locating global ICT facilities.) France, Sweden, and Switzerland comprise the next tier of developed countries with < 100 grams CO2e/KWh.

For more location granularity, one can look at areas within a country. The United States as a whole averages significantly more CO2e/KWh than Europe, so its tiers are a higher. According to the US EIA, Vermont and Idaho averaged <20 grams CO2e/KWh and Oregon and Washington averaged <130 grams. I know of no other country where a an individual state/province can currently offer a CO2e/KWh that is less than 25% of its country's average. Areas participating in Germany's 100%EE program and similar initiatives may be position to do so in a decade or two.

Finally, one could site a green data center just about anywhere if it makes specific provisions to purchase low CO2e/KWh electricity.

It's all in the mix

But there are issues beyond just CO2e/KWh. The strongest responses to the inital version of this post came from Green ICTers concerned about nuclear energy. They point out that nuclear generation requires a non-renewable fuel extracted by mining and generates highly toxic and long-lived waste. Many do not consider it 'clean' or 'green, despite its attractive CO2e/KWh. Sweden and Switzerland generate ~40% of their electricity from nuclear and France over 70%. In the US, nuclear accounts for ~80% of Vermont's electricity but less than 10% in Washington and 0% in Idaho and Oregon.

Another perspective comes from looking at the percentage of electricity generated by renewable energy sources. A New York Times graphic identifies six countries with ~13% or better renewable generation; Iceland and Denmark top the list.

There is even room for discussion on the merits of various renewable sources. Hydroelectricity is 'clean' and renewable, but dams can have a severe impact on both the terrain and the wildlife of rivers, so not everyone considers them 'green'. The New York Times notes that "large-scale hydropower is sometimes excluded from tallies of renewable energy because of its environmental impact." The US states of Idaho, Oregon, and Washington generate 60%-80% of their electricity from hydro, Vermont ~11%. Abundant hydroelectric power in the US Pacific Northwest has made it a popular locale for mega-data centers since 2007.

There are also environment issues associated with wind farms and large solar arrays. Even Geothermal is not without environmental risks, including the possibility of triggering earthquakes, so there is no perfect solution. The best approach is to minimize energy consumption in the first place, then go as clean and green as possible.

Combined Electricity & Heat Generation (2007)

Electricity Generation (CO2e ~2001, Nuclear 2007-10)

Electricity Generation (2009)
Excluding large-scale hydro