I dissagree that Wordpress is not a framework. As a developer you can develop plugins into wordpress that can do some pretty neat things. Okay, you may say that wordpress is not a framework itself, but it does provide a development framework that allows developers to tap into. True it started life as blogging software, but to limit wordpress to only blogs is not doing wordpress the justice it deserves.
Zend_Framework on the other hand is completely different to wordpress. Zend_Framework will help your web development, but it wont do it for you. Where wordpress will work as a content management system/blog out of the box leaving the developer with a plugin framework to allow you to build custom modules and code that will run within wordpress, Zend_Framework will expect you to build everything from scratch. Sure it will provide some helper classes to help you along the way, but its up to the developer to pretty much write their entire application from scratch using Zend tools.
Personally speaking if you want complete and utter control over your entire application, and want to do everything from scratch with some help from Zend, then Zend_Framework is a tool you might want to investigate. If however you just want a website with a little customisation, then wordpress is a very good tool for that.
But the two dont compare side by side, they are different solutions for different problems.