If you've been wanting to get into development with CodeIgniter but are still struggling with some of the first steps, you should check out this new post (and video) from Kenny Meyers that steps you through some of the basics of using the framework and how it handles requests.
Want to know how CodeIgniter works? I mean that question quite literally. What happens when someone visits your site and index.php is loaded by Apache/nginx/php-fpm? I gave a talk at EECI 2010 last year about this and it's finally available. A small note: This was based on the beta of CodeIgniter 2 and since then PHP4 support has been removed as well as a lot of other great features added.