On the symfony blog Fabien Potencier encourages all symfony developers that the security of your application must be taken seriously and that, despite the built-in protection the framework offers, there still could be issues.
The symfony framework has always provided the tools needed by the developers to secure their applications. With the new form framework, we have added an automatic protection against CSRF. Speaking of the form framework, we have also added a lot of security features to protect you against all sort of injections.
He does include an example, though, of a situation where it's not just about protecting from cross-site scripting or attacks. It's about checking user input to ensure it's what it should be. They give the example of a user pushing an "is_admin" value into a form posting where there wasn't one and updating the right column to give them admin rights.
He mentions some work the Rails team has tried to do to prevent this sort of thing automatically, but Fabian points out what the symfony framework already does - prevent any injected fields other than what's in the forms from being submitted and included.