If you separate your business logic from your data access logic, the last thing you want to do is make your business logic unit tests reliant on the database.  This is normally not a big deal: retrieve the data, store it in an array and pass it off to the class with the business logic.  Mocking the data for the unit test simply requires you to hardcode from array information in the test.  However, I recently ran into a case where I wanted to pass Zend_Db_Table_Row and Zend_Db_Table_Row objects to the business logic and mocking them was not so easy.