Infinite scrolling is a really cool effect and a great use of AJAX. It also has some weird side-effects that you'll need to be aware of. This is the starting point I used in the event version of the Gallery Viewer. The major parts are throttling the scroll event and detecting the scrolling direction. Because I am loading images I want to get started on the request as soon as possible, or I would wait until they were close to the bottom of the page before loading. There is a also a little <div> with a message that keeps jumping to the bottom of the page.

    // keep scroll position so we can use it 
    // later to detect scroll direction
    var oldScrollPosition = 0;

    // throttle down the scroll events to once ever 1.5 seconds
    var throttled = _.throttle(function(e) {
        var newScrollPosition = $(this).scrollTop();
        // check if they scrolled down the page
        if(newScrollPosition > oldScrollPosition) {
            self.pullVideoList(self.num);
            oldScrollPosition = newScrollPosition;
        }
    }, 1500);
    $(window).scroll(throttled);