Exemplo mootools getScroll()

O HTML

Scroll to bottom slow

<div id='fader' class="post">If you can see me, you don't need glasses!</div>

O css

body {
    height: 1500px;
}
.post {
    text-align: center;
    opacity: 0;
    position: relative;
    margin-top: 900px;
    height:60px;
    background:#000;
    color: #FFF;
    padding:40px;
}

O JS

var posts;
window.addEvent('domready', function () {
    posts = $$('.post');
})

window.addEvent('scroll', function (e) {
    var altura = window.getScroll().y;
    posts.each(function (post) {
        if (post.getPosition().y > altura + 350) return;
        post.fade('in');

    });
});