As Lukas Smith mentions in his latest post, the Drupal project has made a switch to use the HttpFoundation component of the Symfony2 project as a integral part of their HTTP handling.

[...] The fact that Drupal8 will use Symfony'2 HttpFoundation component is huge, because it will make integration a breeze. Basically Symfony2 converts a Request instance to a Response. So if both Symfony2 and Drupal8 use the same Request and Response classes it means that creating a sub-request and handing the response from Drupal8 into Symfony2 (or vice versa) will require no conversion at all.

He also points out Drupal's use of the ClassLoader component will make integration even easier. This is great news for Symfony fans out there that have been needing to work directly with a Drupal install (or even those wanting to create new Drupal additions in the environment they're familiar with). You'll have to wait for Drupal 8 to get these benefits, though.