function retrieve_zip(callback)
{
	try { if(!google) { google = 0; } } catch(err) { google = 0; } // Stupid Exceptions
	if(navigator.geolocation) // FireFox/HTML5 GeoLocation
	{
		navigator.geolocation.getCurrentPosition(function(position)
		{
			zip_from_latlng(position.coords.latitude,position.coords.longitude,callback);
		});
	}
	else if(google && google.gears) // Google Gears GeoLocation
	{
		var geloc = google.gears.factory.create('beta.geolocation');
		geloc.getPermission();
		geloc.getCurrentPosition(function(position)
		{
			zip_from_latlng(position.latitude,position.longitude,callback);
		},function(err){});
	}
}
function zip_from_latlng(latitude,longitude,callback)
{
	// Setup the Script using Geonames.org's WebService
		var script = document.createElement("script");
		script.src = "http://ws.geonames.org/findNearbyPostalCodesJSON?lat=" + latitude + "&lng=" + longitude + "&callback=" + callback;
	// Run the Script
		document.getElementsByTagName("head")[0].appendChild(script);
}
function example_callback(json)
{
	// Now we have the data!  If you want to just assume it's the 'closest' zipcode, we have that below:
	zip = json.postalCodes[0].postalCode;
	country = json.postalCodes[0].countryCode;
	state = json.postalCodes[0].adminName1;
	county = json.postalCodes[0].adminName2;
	place = json.postalCodes[0].placeName;
	alert(zip);
}
retrieve_zip("example_callback"); // Alert the User's Zipcode

Zip code from HTML5 Geolocation

JavaScript