One of the tales you see circulating in the media every quarter is that the carbon footprint of a Second Life avatar is the same as that of a citizen of Brazil, based on energy consumption. The story goes around a few times each year, though technically it wasn't really all that accurate even when it began to circulate.
Around the time that the calculation was done, Linden Lab was already in the process of slashing power-consumption across its facilities, and presently uses only a third compared to similar servers. Plus, there's the little detail that the carbon footprint of the average Brazilian is quite small, a mere fraction of the world average.
Now the Lab is moving into newer, greener
pastures err ... facilities, having closed a deal with Miami-based Terremark to start moving Second Life servers into Terremark's colocation facilities outside of Washington DC.