Christian Stocker has posted an adapter he's created to hook the Zend_Auth component of the Zend Framework into Twitter's authentication system.

From time to time I write some little scripts for my own needs or just to test out new stuff. And I realised, that I blog about it way too little. So here I start with my latest addition: Zend_Auth_Adapter_Twitter. [...] I came up with a Zend_Auth Adapter for Twitter. The integration was pretty easy, but the code is very basic and rough. Nevertheless, it does the job and alongside the Openid Adapter I can offer a federated login for many people already.

You can grab the code and an example of it in action from their SVN server (as well as this library) or just check out the live demo.