<html>
<head>
<script src="http://maps.googleapis.com/maps/api/js">
</script>
<script>
var map;
function initialize()
{
var mapOpt = {
center:new google.maps.LatLng(51.508742,-0.120850),
zoom:6,
mapTypeId:google.maps.MapTypeId.ROADMAP
};
map=new google.maps.Map(document.getElementById("googleMap"),mapOpt);
}
google.maps.event.addDomListener(window, 'load',initialize);
</script>
</head>
<body>
<button onclick="alert(map.getCenter());">Get center</button>
<br><br>
<div id="googleMap" style="width:400px;height:300px;"></div>
</body>
</html>