Do you check all users input before sending it to the DB? A good website to look at, that covers some good areas of security is:
http://phpsec.org/projects/guide/
This guide will give you examples of common coding mistakes, so I would recommend puting your website on a testing location. and point that tester site to a test copy of your db. Then try to destroy your own website using given the examples. I would not do any of the tests on your live site.
Also I would reccomend PHPClasses.org this is a repository of opensource php classes, many of them are for security: http://psbweb.mirrors.phpclasses.org/browse/class/78.html
Also I would suggest signing up for this newsletter:
http://www.securityspace.com
They send security alerts via email, it is free for the email. So you can check all of the newest security issues. Since you are using PHPBB, you need to check there site ofter for security updates, I personaly use there forum in a few sites, but as you know it is open source, this by default makes it some what of a risk as millions of people have the complete source code, so people find flaws in the code and exploit those flaws from time to time.
By the way I like the site, verry clean and easy to navigate!
Chris