On the Symfony blog today Fabien Potencier answers the question "will Symfony 2.0 finally use PHP 5.3?"

Both the upcoming Zend Framework and CakePHP 2.0 versions will rely on PHP 5.3. And for Symfony, I said it will still be compatible with PHP 5.2. From my point of view at the time, it would be a mistake to upgrade major frameworks to PHP 5.3 now for one main reason: major frameworks are used by many big companies, and upgrading to the latest version of a software fast in such companies is not always feasible.

According to Fabien, the Symfony team has changed their mind about the PHP 5.2 support, though - they're making the leap directly to PHP 5.3 and will be dropping the 5.2 support. The second half of the post gets into the "why" part of the decision (mostly about integration with current tools and helping to further the PHP community's support/adoption of PHP 5.3).