On NETTUTS.com there's a new tutorial they've created showing the development of a "load more" component for your page combining PHP, MySQL and a javascript library (like Mootools or jQuery).

Both Twitter and the Apple App Store use a brilliant technique for loading more information; you click the link and fresh items magically appear on the screen. This tutorial teaches you to use AJAX, CSS, Javascript, JSON, PHP, and HTML to create that magic. This tutorial will also feature both jQuery and MooTools versions of the script.

The idea is that, after the initial load of the page, the user can click on the "load more" link/button and the Ajax request will get the latest results from the backend (via json messaging). The complete code is available for download and you can check out the demo if you'd like to see it in action.