New on the Developer.com website is this tutorial from Jason Gilmore looking at OpenID authentication through the Zend Framework's Zend_Auth component.

I'll show you how to take advantage of a powerful open user identification service known as OpenID, and show you how to incorporate OpenID authentication and identification features into your Zend Framework-powered website. Even if you haven't started taking advantage of a web framework such as Zend's, this tutorial will serve two important purposes, demonstrating not only the allure of OpenID, but also how a framework such as the Zend Framework can greatly reduce development time.

He points to several places you can set up an OpenID account and mentions the Zend_Auth component that already includes all of the methods you'll need to validate a user based on their choice of provider. He creates a simple form and makes an indexAction in his controller that makes a Zend_OpenId_Consumer object to grab the user's information and verify the login.