Lat/Lon are already [unused] columns in the groups table if we can get them added to the users table (still unused for now) we could use tools like Tony's geocoder to populate them and then begin to show "groups near you" and "members near you" with custom modules.

Proximity searchs are possible now using the geocoder, it comes with some custom sql functions to enable proximity in a sql query.
I have used Netisms Satellite module to query and display such lists.

See http://www.fatgeorge.co.uk/Community.aspx

You need to be logged in and have been registered for more than 5 mins (geocoder runs every 5 mins).
Your user profile needs to have been geocoded, in your profile are some proximity entries, the defaults are distance = 500 and units = miles.

see the Groups near me and members near me, depending on where you are you may need to increase the proximity distance in your profile settings in order to see anything.