Fabien Potencier has recently posted a tutorial over on the symfony blog about using cross-application links in your application:

A symfony project is made of one or more applications. Applications share nothing, but the model classes. But, even if the symfony documentation is crystal clear on the subject, many developers want to share more than just the model. The most requested feature being the ability to create links to a frontend application from a backend one.

This sort of linking is relatively simple in the 1.1 release of the framework - its instances of sfRoutingConfigHandler and sfPatternRouting that let you create the configuration classes (and code) to grab information and functionality from one application to another.