On the Symfony-framework.com website, there's a look at what's new in the latest Symfony build, version 1.1, as presented by Fabien Potencier at this year's International PHP Conference 2007.
In this article, I will briefly explain all the significant changes of Symfony 1.1 so that you will have fewer doubts deciding between: rewrite part of your old code, simply update your application or use the new Symfony 1.1 only in new developments.
There's three things he mentions - the new task framework to help organize code development, the reorganization of the event dispatcher and its dependencies, and the new Form, Widget and Validator framework.