A SecondLife avatar has approximately the same carbon footprint as a Brazillian

"If there are on average between 10,000 and 15,000 avatars "living" in Second Life at any point, that means the world has a population of about 12,500. Supporting those 12,500 avatars requires 4,000 servers as well as the 12,500 PCs the avatars' physical alter egos are using. Conservatively, a PC consumes 120 watts and a server consumes 200 watts. Throw in another 50 watts per server for data-center air conditioning. So, on a daily basis, overall Second Life power consumption equals:

(4,000 x 250 x 24) + (12,500 x 120 x 24) = 60,000,000 watt-hours or 60,000 kilowatt-hours

Per capita, that's:

60,000 / 12,500 = 4.8 kWh

Which, annualized, gives us 1,752 kWh. So an avatar consumes 1,752 kWh per year. By comparison, the average human, on a worldwide basis, consumes 2,436 kWh per year. So there you have it: an avatar consumes a bit less energy than a real person, though they're in the same ballpark.
Now, if we limit the comparison to developed countries, where per-capita energy consumption is 7,702 kWh a year, the avatars appear considerably less energy hungry than the humans. But if we look at developing countries, where per-capita consumption is 1,015 kWh, we find that avatars burn through considerably more electricity than people do.
More narrowly still, the average citizen of Brazil consumes 1,884 kWh, which, given the fact that my avatar estimate was rough and conservative, means that your average Second Life avatar consumes about as much electricity as your average Brazilian."
Source: Nicholas Carr, 5th December 2006, on his blog Rough Type