Matthew Weier O'Phinney (of the Zend Framework project) had an opportunity to attend the Symfony Live event held in Paris and presented on integrating the two frameworks (usually seen as rivals by much of the PHP community).
To be honest, I was a little worried about the conference -- many see Symfony and ZF as being in competition, and that there would be no cross-pollination. I'm hoping that between Fabien, Stefan, and myself, we helped dispel that myth this week. The fact of the matter is that no single project can be fully comprehensive, and do everything perfectly.
He talks a bit more about the real difference between most of the MVC frameworks out there (tooling and communities) and each framework's development goals. Matthew's goal was to show Symfony developers that it's not just about staying within the ecosystem of your selected framework when developing - you need to branch out and find the right tool for the job. Sometimes that's found in the Zend Framework, other times it's elsewhere.