Ivan Iordanov has posted a method he's come up with to extend the existing Zend_Db_Table functionality (of the Zend Framework) to create NestedSet models for your app.

Last days I've been trying to customize Zend's Framework. My goal is to create simpler models. Currently I'm making some db models that handle stuff around wide used designs. Such design is the Nested Set model. It is an adjacent list realized in SQL as a tree. You can find a good introduction to nested sets at mysql developer zone.

He steps through the creation of some example database tables, his class and controller to hook into the Zend Framework application and this full code for his interface class to make the NestedSet models easy.