Jani Hartikainen has posted his idea for using forms along with models in a Zend Framework application.
Matthew Weier O'Phinney wrote about using Zend_Form with models. His approach puts a form inside a model, which then uses the form to validate itself. While this idea is not bad, I find it being upside down - I think the form should use the model to validate itself, not the other way around.
Jani describes two alternate methods - using a global model to do the checking or a model-based validation class that would inherit the model's functionality through being extended. He describes each option's setup and potential use (no code is included save for a few small examples).