with the changes that have gone in 2007 i wonder if the notion of "4 billion" has gone. the "4 billion" of course was the number of people that the SL and SL-like would serve in time. with or without SL, we need to keep going and eventual bring in those 4 billion or more.
i have start a board called sim.mischiefworks.net that tries to close the gap a little. it is based on opensim and realXtend, although now realXtend has become part of opensim. there are stand alone servers, grid servers, and hypergrid servers. of course it not as good as SL itself, but they are come along.
my approach is a little different. it use opensim + npcs. ncp == non player characters. the site is just open and fortunately for you, i cant type for very long. so you will be able to get your responses in just fine.
so basically it is SL, opensim/realXtend, and EQemu. the site is
sim.mischiefworks.net