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Here is how you can have Google Maps automatically center at the user's location based on their IP Address.

First, download the latest free GeoCity Lite Binary data from here: http://www.maxmind.com/app/geolitecity (click the link that says Download the latest GeoLite City Binary Format)
Next, download the PHP Pure APIs from http://www.maxmind.com/app/php. The files you want to snag are geoipcity.inc and geoipregionvars.php.

Couple of things to mention: