In this post to DZone.com today, John Esposito points out a site that compares popular PHP frameworks socially, a collaborative resource to keep track of things like framework features, popularity and their last update.
There's this wiki-ish comparison of PHP frameworks from socialcompare.com -- presently incomplete, but already containing quite a few frameworks and interesting details of each. You can add comparison criteria (rows; folksonomically) or update existing criteria -- which currently include, among other details, ORM specifics, cache storage, template system, and testing libraries.
Frameworks currently covered include Zend Framework, CakePHP, Fat-Free, Symfony, Yii and Kohana. As it stands now, Yii is the most popular framework with 8 votes and Symfony and Zend Framework are tied for second with 5 points each.