On the surface it sounds like a good idea but...
Actually it could be done with Google maps API but it would not be easy. The issue is you would not want everyone who joins the site to get a Push Pin on the map only active users. Why only active members because this site has 21,712 as of right now. That number will go up or down slightly over the next couple of days but over a years time will only continue to grow. If we push pinned every member as they join or allow people to add their own pin sooner than later you would blanket the whole earth with members depending on your zoom level.
Now you would have to define Active Users most likely that would be based on post count and a time to live for post count. Meaning you have 10 posts so you are an active member but over the course of a year you post no more so your pin is removed.
Then each group would want their own custom push pin so you would have to pick a forum association in your profile.
Also to get your push pin on the map your profile will have to list a location for you. This way it could all be handled by scripts and a kml file. Of course some people don't like to reveal where they live (rightly so) so they do not enter their location they would not get a pin.
Each push pin could direct you to each members SOH home page if you set it up right.
I would not want one that just allowed people to add pins anywhere they want for this place it would be a nuts.
But of course in just a few short sentences I created a programming nightmare that is all doable.
Who wants to volunteer to write the code?