On NetTuts.com there's a new post of a two-part screencast series helping you get authentication working in CodeIgniter based on the information in a MySQL database.
In this in depth “screencast of the week,” I’ll take you through the process of building an authentication system with CodeIgniter. Need to restrict access to certain parts of your website to only those who’ve logged in? This tutorial will teach you how!
There's two videos in the series (both included in the post, but with links to the full versions) and the sample code is included as a download. The cover using the form helper that comes bundled with CodeIgniter to create a basic form that validates off of the user's information in the backend database tables.