The Guardian News Blog has a post about 
the estimated energy consumption of a Second Life avatar.  Further to my previous post about 
Carbon-Adjusted Computation, can I give out a call for all web service providers running computers on my behalf to provide these kinds of stats?  It's really important that these things are visible.
Update: there's an insightful comment from fred2 on the Guardian blog post that in my opinion provides a better estimate, roughly a third of the first one.à The point here isn't the actual numbers, it's that we're thinking about the numbers at all.