I got a geeky urge, and played around with the SL statistics for May 2007. I added country population (July 2007 estimates, according to the CIA World Factbook) for the top part of the list as sorted by number of avatars, as well as selected countries from lower down the list.
Antarctica, 211 active avatars (who knew they would keep statistics?), is number one with over 5%, based on the peak summer estimate of 4,000 residents. If based on the winter population estimate of 1,000, the percent would increase to a whopping 21%. American Samoa, 419 active avatars, comes in with 0.73% of a population of 57,663. Netherlands, 17,130 active avatars, is third at 0.10% of a population of 16,570,613.
The United States, predictably, drops from 1 (130,033 active avatars) to 19 (only 0.04% of 301,139.947). Most European countries at the top of the avatar numbers move ahead of the US.
Here’s the abbreviated list sorted by % of population (google docs).