Starting with symfony 1.1, it is now possible to easily use Propel 1.3 in your project to take advantage of its speed improvements, nested set implementation, object instance pooling, among others. Most importantly, Propel 1.3 uses PDO instead of Creole as the DBAL, offering a significant performance boost.
Last week, we've fixed a security bug allowing XSS attacks in certain circumstances. The related ticket was opened more than a year ago.
Yahoo! used symfony to redevelop another project. This time symfony was used as part of the foundation for Yahoo! Answers.