The Developer Tutorials blog has an interesting perspective on the whole framework front today - use whatever framework you choose, but be sure to "add a little Zend" when you need it.
The Zend Framework is a fairly standard, (optionally) MVC PHP application framework. It comes with all the usual functionality; request routing, database access, templates (through view files) etc. [...] But here's the kicker: it works entirely standalone. The classes and their methods can, generally, be used statically in any context, or at least independently of the framework.
They point out that the components of the Zend Framework, while able to happily live in their self-contained MVC bubble, can also break "out of the box" and live their own happy, separate lives. He even points out another blog entry looking at using the Zend Framework right along side CodeIgniter in a web application.