In a recent presentation at Google, Philip Rosedale and Cory Ondrejka of Linden Lab revealed a fresh batch of statistics related to the server and client side of Second Life, Linden Lab's virtual world. Note that the stats were cited conversationally and not in a prepared statement:

There are 2,000 "simulators" used for Second Life, constituing 125 square kilometers of virtual reality.
25% of Second Life's users are from outside the USA, with UK the second-largest country of origin.
The average age of users is 32, and the median age of users is 36.
43% of users are female, 50% are female "by use."
Linden Lab stores 15 terrabytes of user-created data on its servers.
There are $5 million USD in virtual transactions per month conducted between users.
180,000 distinct objects were sold in a 30-day period roughly spanning Feburary, 2006.
75% of users are buyers, 25% are sellers.
"low hundreds" of members call Second Life their full-time job
Anshe Chung, Second Life's most notable land baron, makes $175k per year flipping virtual real estate.