Test in JQuery if an element is at the top of screen

var distance = $('div').offset().top,
    $window = $(window);

$window.scroll(function() {
    if ( $window.scrollTop() >= distance ) {
        // Your div has reached the top
    }
});

P.S. For better performance, you should probably throttle the scroll event handler.
Check out John Resig’s article: Learning from Twitter.