If you should get the following error messages:

could not connect to server: Permission denied (PGError) Is the server running locally and accepting connections on Unix domain socket "/var/pgsql_socket/.s.PGSQL.5432"?

and/or

FATAL: lock file "postmaster.pid" already exists HINT: Is another postmaster (PID 592) running in data directory "/usr/local/var/postgres"?

Try simply uninstalling gem uninstall pg and reinstalling gem install pg the pg gem. Doing so set things right for me.

This is on OS X Lion with the homebrew postgres package. I believe I got the error because when I updated my pg gem it somehow didn’t pick up the path to my homebrew postgres and tried to connect to the system one instead, but I haven’t done enough testing to know for sure. At any rate, the problem is solved now, so my curiosity about the cause of it has waned.