Will points to this back of the envelope calculation of the amount of electricity a Second Life avatar consumes, which is interesting in itself, but look at this:
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.
Um... so on average each server supports, say, 3-5 users? This kind of puts the promise to open source Second Life in another light, particularly for K-12. What kind of horsepower would it take to run a Second Life node that would allow, say, 50 students to work in at the same time? If it is more than one beefy new server, I don't see it happening.