Benjamin Eberlei has posted some of the results of his work at decoupling Symfony security from the FOSUserBundle, one of the "Friends of Symfony" packages that provides easy user management in your application.
In this blog post I will show how to decouple your core application from the Symfony Security component and User bundles such as the FOSUserBundle. [...] This blog post will add another perspective on how to achieve decoupling from Symfony user and security with a very simple approach. With this puzzle piece and others I wrote about before (Controllers as Service, Param Converters) and other pieces I still need to write about, I would classify Symfony2 as a good framework.
He talks about some of the dependencies this bundle introduces and one way to mitigate it by extending the User object with your own custom entities. His example is a "Customer" entity that only defines the "id" and "username" properties.