Database security is essential with today’s web applications. You should have the control, not the bad guys that are visiting your website. Exposing sensitive information from your database is bad, loosing data from your customers is even worse.
Robert Bernier published an article plus downloadable PDF on IBM’s developerWorks where you can learn about security measures to protect your PostgreSQL database.
Update 2020-01-18:
Replaced the link since it was no longer working. Check also his presentation PostgreSQL: Welcome To Total Security on SlideShare!