Robert Basic has posted an example of the use of the Zend_Auth component of the Zend Framework inside of an example controller.
So, here's what I'm going to do: authenticate an user against a database table using Zend Framework's Zend_Auth component. It's really a piece of cake. You can see a working example here: http://robertbasic.com/dev/login/.
He sets up a registry instance, a database table for the logins and the sample controller with both login and logout functionality. Complete code (and links to Zend Framework documentation are included).