Steps to uninstall PostgreSQL 8.4 (Assuming default locations)
1) Stop the server and remove the service.
sudo launchctl unload /Library/LaunchDaemons/com.edb.launchd.postgresql-8.4.plist
2) Remove the plist file.
sudo rm -f /Library/LaunchDaemons/com.edb.launchd.postgresql-8.4.plist
3) Remove the Menu Icons
sudo rm -rf /Applications/PostgreSQL/8.4
4) Remove the Installation Files.
sudo rm -rf /Library/PostgreSQL/8.4
5) Remove the postgres user.
sudo dscl . delete /Users/postgres
6) Remove the ini file
sudo rm -f /etc/postgres-reg.ini
This will uninstall PostgreSQL 8.4 cleanly from your mac.
