Robert Basic found the creation of a CAPTCHA with the Zend_Captcha and Zend_Form components of the Zend Framework "a bit tricky" and decided to share an example he's come up with to help others out there.
I'll just show a quick example how to implement Zend_Captcha into a Zend_Form, may be useful for someone. There are several CAPTCHA types in ZF, like the Image, Figlet and Dumb. I use Image.
He includes the code for his form where the CAPTCHA component is configured to use a certain directories for the location to put the images, the font to use, and the label to put on it. He sets up the action for it and creates a captchaAction method in the same controller. This does the validation on the entered term to ensure that it matches the string in the image exactly.