It's not only Symfony2, PHP has a lot of high-quality projects going on: Zend Framework, Doctrine, Solar/Aura, Twig, Lithium, Jackalope, FLOW3 and probably much, much more.
While some of your other listed projects are okay (as it happens, Doctrine and Twig are both used as part of Symfony2), Zend is generally considered about as ancient as CakePHP today, and I don't really know anyone who recommends using it for new projects. Most of the "big-name" PHP projects out there are creeping horrors (I'm looking at you, Wordpress), and Zend kinda falls into that bucket.
Personally I'd be very wary of Lithium as the code looks pretty slipshod internally from a brief scan into it, but Solar and FLOW3 are solid projects.
ZF is from a totally different league than CakePHP or Wordpress. Yes, a bit ancient today but in many parts it's a solid, well-written, flexible and extensible code well-suited for more "enterprise" needs. Probably more "production-ready" than Symfony2 is at the moment. Plus: ZF2, which will require PHP5.3, is in the works so it should make the project more "relevant" again.
Didn't dwell too much into Lithium's code but saw they have some interesting ideas also based on 5.3 features, e.g. they use a sort of aspect-oriented programming based on closures and lambdas. Sounds cool, even if only for experimenting with different programming approaches.
ZF2 may fix a lot of the complaints I (and other folks) have with Zend, but right now it's really hard to recommend it.
It's actually pretty much in the same league as CakePHP: old and slow. Not much to recommend it right now. (Symfony2 actually can pull in Zend-based libraries on its own without a problem, which is why among the more sarcastic of us I've heard Symfony2 described as "Zend 2 but actually here".)
Well, Symfony2 itself only until very recently was using some of the ZendFramework libraries internally (and Fabien often praised ZF), which I would say comes as a proof that it is a solid code, even if it's obviously lacking in some parts comparing to younger alternatives. In many cases it's still the best thing in the market though (Zend_Search_Lucene, Zend_Pdf, Zend_Amf, Zend_Gdata come to mind).
Personally I'd be very wary of Lithium as the code looks pretty slipshod internally from a brief scan into it, but Solar and FLOW3 are solid projects.