Ray Cheung posted his comparison of the Zend Framework and CakePHP a little while back to share some of the thoughts and research he did to pick one over the other for his application.

A lot of people have been asking for some comparison articles for some of the popular frameworks. [...] Overall, both these frameworks are good to use as per your requirements. CakePHP is suitable for developers new to MVC and those who require stricter conventions. ZF is a choice for those who need better control over the application design and built-in support for renowned Web services. Both the frameworks are developing fast and the choice is entirely yours.

He includes brief overviews of each framework and comments on documentation and configuration issues surrounding them.

Commentors on the post include Nate Abele (of CakePHP) and Wil Sinclair (of the Zend Framework).