I'm new to WordPress, but I've been looking for a blog solution to integrate with phpBB for a while. There are a number of people who are rolling their own blogging system, but I find that WP has some great features. I checked out serendipity, but it wasn't easily integrated. Luckily, I think WP can be intergrated with phpBB fairly easily.
I have started this process by getting WP to use phpBB's user table. There are still a bunch of bugs, but you can login and post things. The user management stuff in WP doesn't really work (you would probably use phpBB in this case anyway).
I also develop over at http://www.mx-system.com In addition to making WP phpBB compliant, I'm going to break it apart into blocks for MX-System. MX-System is a modular portal for phpBB.
Here's the URL for the very early WP that supports phpBB logins: http://www.gortbusters.org/mx-system/wp-phpbb.tar.gz