Here's an example: http://duncan99.wordpress.com/2011/10/08/google-maps-api-infowindows/
marker.addListener('mouseover', function() {
infowindow.open(map, this);
});
// assuming you also want to hide the infowindow when user mouses-out
marker.addListener('mouseout', function() {
infowindow.close();
});