Although this question has already been answered, I think this approach is better : http://jsfiddle.net/kjy112/3CvaD/ extract from this question on StackOverFlow google maps - open marker infowindow given the coordinates:
Each marker gets an "infowindow" entry :
function createMarker(lat, lon, html) {
var newmarker = new google.maps.Marker({
position: new google.maps.LatLng(lat, lon),
map: map,
title: html
});
newmarker['infowindow'] = new google.maps.InfoWindow({
content: html
});
google.maps.event.addListener(newmarker, 'mouseover', function() {
this['infowindow'].open(map, this);
});
}