With the recent stir caused by the new iPhone, it's only natural that developers are looking for easy ways to make their sites as compatible as possible with the mobile platform. The Symfony project has posted a guide to helping you make your apps optimized for mobile use.
symfony 1.1 introduces native support for different formats and mime-types. This means that the same model and controller can have different templates based on the requested format. The default format is still HTML but symfony supports several other formats out of the box as defined in the factories.yml file.
Using this system, it's easy to add new output formats by adding to the default layouts the framework comes equipped with. The Content-Type can be changed and layouts can be disabled as needed. This lets you make a custom definition just for use with the iPhone without a lot of hassle.
They also include an example of how to make one of these optimized layouts, including some iPhone "screenshots" of the resulting pages.