WP-United integrates phpBB and WordPress to make a social site. Enable any of these modules: sign-on, theming, widgets, cross-posting and behaviour.
Perfect plugin to bridge WP and phpBB. Plugin creates a seamless transition from WP to phpBB. It took a bit to find automod and WP-United-phpBB module required for the plugin to work. The mods and phpBB package files are located here http://www.wpzoan.com/wp-united-forum-plugin-a-wordpress-phpbb-bridge/ . Everything you'll need to get the plugin up and running