How do I automatically find a user's location?

Here's a link to the documentation on the relevant Google Maps API:

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

It shows an example of how to use it:

/** * Set the currentState_ and currentCountry_ properties based on the client's * current location (when available and in the US), or to the defaults. */InTheNews.prototype.setDefaultLocation_ = function() {  this.currentState_ = this.options_.startingState;  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 United States. range    // check the region so that we can safely use it when selecting a    // state level polygon overlay    var state = google.loader.ClientLocation.address.region.toUpperCase();    if (InTheNews.stateNames[state]) {      this.currentState_ = state;    }  }  this.currentCountry_ = "US";}

And tells you what you will get from it:

When populated, the google.loader.ClientLocation object is populated with the following metro-level granularity properties:

• ClientLocation.latitude — supplies the low resolution latitude associated with the client's IP address

• ClientLocation.longitude — supplies the low resolution longitude associated with the client's IP address

• ClientLocation.address.city — supplies the name of the city associated with the client's IP address

• ClientLocation.address.country — supplies the name of the country associated with the client's IP address

• ClientLocation.address.country_code — supplies the name of the ISO 3166-1 country code associated with the client's IP address

• ClientLocation.address.region — supplies the country specific region name associated with the client's IP address

They usually do using IP resolution by IP tables. ip2nation might give some idea

You might like to look at MaxMind's GeoLite soluton for IP location. An open source solution.