Agreed. Symfony2 is an excellent base for this reason--Fabien Potencier and friends are, as far as I'm concerned, the guys to look at for writing clear, maintainable, excellent PHP code.
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.
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.
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".)
check benchmark: http://www.yiiframework.com/performance/
Symfony2, on the other hand, puts a fairly strong emphasis on security, is fully based around PHP 5.3 idioms, and is written with sparkling code.
[1] - http://dev.kohanaframework.org/issues/2766 comes immediately to mind; forget the resolution of the bug (such as it wasn't), the behavior of the developers is not good. I get the feeling from those who've used Kohana that this isn't a unique situation.