methods for seeing if something is in the viewport. see also http://github.com/xing/prototype-within-viewport

prototype-within-viewport.js

Element.Methods.centerIsWithinViewport = function(element) {
    var dim = document.viewport.getDimensions();
    var so = document.viewport.getScrollOffsets();
    var co = element.cumulativeOffset();
    var edim = element.getDimensions();
    // x and y measured from upper left
    var center = {
        x: co.left + (edim.width / 2),
        y: co.top + (edim.height / 2)
    }
    return (center.x > so.left &&
        center.x < so.left + dim.width &&
        center.y > so.top &&
        center.y < so.top + dim.height);
};
Element.Methods.isWithinViewport = function(element) {
    var dim = document.viewport.getDimensions();
    var so = document.viewport.getScrollOffsets();
    var co = element.cumulativeOffset();
    var edim = element.getDimensions();    
    // there exists one corner within the viewport
    var ul = { x: co.left, y: co.top };
    var ur = { x: co.left + edim.width, y: co.top };
    var ll = { x: co.left, y: co.top + edim.height };
    var lr = { x: co.left + edim.width, y: co.top + edim.height };
    return (ul.x > so.left &&
        ul.x < so.left + dim.width &&
        ul.y > so.top &&
        ul.y < so.top + dim.height) ||
        (ur.x > so.left &&
        ur.x < so.left + dim.width &&
        ur.y > so.top &&
        ur.y < so.top + dim.height) ||
        (ll.x > so.left &&
        ll.x < so.left + dim.width &&
        ll.y > so.top &&
        ll.y < so.top + dim.height) ||
        (lr.x > so.left &&
        lr.x < so.left + dim.width &&
        lr.y > so.top &&
        lr.y < so.top + dim.height);
}
Element.addMethods();

// then use a periodical executer to see if they scroll it into view
// now you know
document.observe("dom:loaded", function() {
    var e = $('test');
    new PeriodicalExecuter(function(pe) {
        if (e.centerIsWithinViewport()) {
            pe.stop();
            alert('made it in view')
            // log it so we know people are at least seeing the message
        }
    }, 3);  
})