In this post from forumtools.blogspot.com, there's information on how to integrate the Google Maps functionality into your pre-existing phpBB forums.
The integration relies on this set of scripts, a mapping application that lets users of the forums note their favorite locations in a simple interface.
The Geoforum package uses phpBB as a base, integrating simply into its structure. Users can even start discussions/topics surrounding a specific location. There are all sorts of features in the package, so if it sounds like something you've been looking for, be sure to check it out.