The way it works is that the author can specify whether or not he thinks it's threadsafe (yes/no/maybe), which is then verified by users who can specify whether they agree. If a user marks a plugin as not-threadsafe (or not-Rails 3, or not-JRuby, or not-Ruby 1.9), the author has 7 days to help the user come around before it sticks.
So far, there are 60 plugins marked as threadsafe (which means that either the author said "yes" and nobody disagreed, or the author said "maybe" and all the votes so far say yes).
I found that the postgres gem, the most immediately important for me, isn't listed. Do I need to be the gem author/maintainer to register a gem, or can I just add it?
As a psql fan, it makes me wonder what the rails world would be like had dhh had picked psql from the start.