Foren / Craftwork / Server-side / Radius search

I do I do, well sorta to a point. It just involves the normal geoip/geocode api and coomparing the lat/lon against the stored.

Added 5 minutes later:

While this is for a store locator, this is the exact basic of what you want.

http://code.google.com/apis/maps/articles/phpsqlsearch.html

It has everything you need to do what you want.