Form and Validation components were heavily updated this week, including the refactorization of code to a new Locale component and the implementation of new fields and constraints. In addition, Doctrine project published the first stable release of Doctrine 2.0.
Symfony2 committed this week two huge changesets related to the Security component: fixed user refreshing after unserialization and added authentication trust resolver. It also committed two small but important tweaks: the new hash syntax for Twig templates and the new format for XML attribute names.
This week, the Symfony2 Preview Release 4 version was released. Despite being far from finished, this is a full-featured version and therefore it's the best way to learn the new Symfony2 philosophy.