developertutorials.com has a post which highlights a recent performance analysis of Zend, Cake, and CogeIgniter. The concluding sentence is a good summation:
...if you’re building small applications, CakePHP will clearly save you time in development, CodeIgniter will offer massive performance benefits, and Zend will give you a reasonable middle ground.
Still not sure which one I want to try, but I lean a bit toward Zend (although it has received some recent criticism regarding its coding conventions and documentation). I like it that you can use it as a full-blown framework or just import the features you need for your application.
When I get some time, I guess *shrug*