[phyrrus9]

I actually already wrote a web application that did this for another forum. Maybe I'll bring it back.

Do it! I'd really like to see which members are within range. If we see trends we might even be able to do a bigger gathering.

It worked based on geo-IP (which is not terribly accurate but that was the point after all), and I'm not sure if members are too keen on sharing that information with me

Make a simple PHP script that prompts them to share location data, geocode it, and then call it good. An IP address doesn't mean anything, if a member is too scared to share it, they probably don't know a whole lot about tech.