The waypoints plugin covers this and more.
http://imakewebthings.com/jquery-waypoints/
Docs: http://imakewebthings.com/jquery-waypoints/#docs
eg:
$('.someSelector').waypoint(function(direction){
//do stuff
},{
//bottom-in-view will ensure event is thrown when the element's bottom crosses
//bottom of viewport.
offset: 'bottom-in-view'
});