I was asked to do something like this today. A fairly easy way to do it is to create the map of users on a WordPress site with an exising map plugin, and then use a custom onebox to embed it on your forum. I wrote a couple of functions in WordPress to add markers and some basic user-info to a map when the user fills out a ‘location’ field on their WordPress profile. I copied a onebox plugin that @fefrei posted on here yesterday to embed the map on the a forum.
With some changes that are being added to the wp-discourse plugin by @iamntz, it will soon be possible to use Discourse as an SSO provider for a WordPress site. Using that, it could be possible to set something up where one WordPress site provides a service like this to a number of forums.
This particular map plugin is quite good. It can potentially provide a lot of information.
