Do you use MacPorts? Ever switch from one version of PostgresQL to another, e.g., 8.4 to 9.0? Did you notice that it started complaining about a non-existent home directory for the postgres system user? Me, too.
My first instinct was to edit the user, but there is no entry for postgres in /etc/passwd. That led me to this excellent article. Using that info, I cooked up the following command:
sudo dscl localhost change /Local/Default/Users/postgres NFSHomeDirectory /opt/local/var/db/postgresql84 /opt/local/var/db/postgresql90
Back to flipping out...