Per the discussion in [policy, no patch] Follow symfony 2.7 or 3.0., this is the question, we decided that Drupal 8.0 would use Symfony 2.7, and that we would upgrade the Drupal 8 beta to the Symfony 2.7 beta when it came out so that we can get a headstart on determining if there would be any issues. We would then continue to upgrade through the Symfony 2.7 beta cycles and through production releases, etc.
Symfony 2.7 was released on 11 April 2015. Symfony 2.7.0-BETA1 released. This link also has a list of the most significant changes, as well as a link to the full changelog.
This is a minor upgrade and according to [meta] Ensure vendor (PHP) libraries are on latest stable release it is therefore a Critical priority. In particular, as Symfony 2.7 will be a LTS release, we should get it in to Drupal 8 as soon as possible.
We should note that the Symfony folks have changed their strategy slightly. Rather than going from 2.7 to 3.0, they will also have an additional 2.8 release which will come out at the same time as Symfony 3.0 in November, Transition from Symfony 2.7 to 3.0... Symfony 2.8 on its way. The Symfony 2.8 release will also be a LTS release like Symfony 2.7, so we may want to consider as we approach November whether we want to upgrade to Symfony 2.8. However, we do not need to make that decision at this time.