In response to a question on stackoverflow, here is a short guide on how to completely uninstall PostgreSQL 9.1 from Mac OS X Lion.

1. Open a terminal window

   Utilities->Terminal

2. Run the uninstaller

   sudo /Library/PostgreSQL/9.1/uninstall-postgresql.app/Contents/MacOS/installbuilder.sh

   

   

   

3. Remove the PostgreSQL and data folders (Wizard will notify you that these were not removed)

   sudo rm -rf /Library/PostgreSQL

4. Remove the ini file

   sudo rm /etc/postgres-reg.ini

5. Remove the PostgreSQL user

   System Preferences -> Users & Groups

   Unlock the settings panel by clicking on the padlock and enter your password

   Select the PostgreSQL user and click on the minus button.