Leaving the air-conditioning needs out of it right now and without knowing the exact number of servers needed to run SL with 4 regions per server it is still a huge amount of Electricity. However, that they are moving to a Data Centre powered by nuclear power certainly backs up the claim of lowering the carbon footprint on their side.

But, you still have a constant minimum 40,000 computers connecting to the grid(tm) each of those using 1,752,000kWh of electricty, which based on coal fired technology is a minimum of... 54,312,000 short tons of CO2 for the minimum 40,000 players of Second Life.

For more tangental calculations... Yes, I am now venturing into the ridiculous >.>)

54 million tons of CO2 for SL's 40,000 campers. 10 linden per hour by 40,000 people is 3.5 billion $Linden. In other words each $Linden has an environmental cost of 31 pounds (14kg) of CO2,

Buy US$20 of $Linden and you just helped to create 84,000kg of CO2