I like this intuitive user experience. I have previously seen this on Microsoft Live image search and on a few other sites as well.

It seems to be ideal for cases where the top-most results are the ones most-likely to be pertinent to the user such as search results or ordered lists. It also would be nice for twitter or facebook-like timelines.

I did a quick test and it does not seem to work in Safari on the iPhone. The issue here might have something to do with $(window).scroll event not being compatible with this browser, although $(document).scroll ought to be.

Quirksmode scroll event info: http://www.quirksmode.org/dom/events/scroll.html