if (GBrowserIsCompatible()) {
    var map = new google.maps.Map2(document.getElementById("my-googlemap"));
    if (google.loader.ClientLocation &&
        google.loader.ClientLocation.address.country_code == "US" &&
        google.loader.ClientLocation.address.region) {
        // geo locate was successful and user is in the states
        map.setCenter(
            new google.maps.LatLng(google.loader.ClientLocation.latitude,
                                   google.loader.ClientLocation.longitude
                                   ), 8
            );
        // do the rest of your map stuff here
    }
}

http://code.google.com/apis/ajax/documentation/#ClientLocation

Set the center of the map on the person's location based on IP address.  In this case the user is required to be in the USA.

google.loader.ClientLocation to get a person's lat/long using their IP address

google, ip

JavaScript