IPtoLL is a hostname to latitude/longitude conversion tool (perl module) that uses whois and the DBI module to interface with a relational database (like SQL) in which a domain-to-city cache and city-to-location database are stored. Not perfect but it does work suprisingly well.

http://www-unix.mcs.anl.gov/~olson/IPtoLL.html

Update: You can't automatically determine the general geographic location of a visitor with greater than national resolution. What you can do is determine the address listed in the DNS record that may or may not correspond to the general location of the box the visitor is accessing your site with. Most of the time the user is in the same country the request came from, if the DNS record is correct.

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