Using the IP address to pre-populate a form

I see some sites ads list my location (city, state, and Country) using my IP address. I would like to access this information and pre-populate the form.

Has anyone here ever done this?

Found a free data base and script here for the country... which is what I think I'll limit it to.
http://software77.net/cgi-bin/ip-country/geo-ip.pl

ug - dunno if I'll do this at all. It took 22 seconds to find the right country