For people who are looking for which version of Symfony2 we have to use to keep compatibility and to keep the progress of the staff community, here is the screenshot (cf symfony2 site) of the version:
Yellow represents the Development phase
Blue represents the Stabilisation phase
Green represents the Maintenance period
This results in very predictable dates and maintenance periods.
(special) Symfony 2.2 will be released at the end of February 2013;
(special) Symfony 2.3 (the first LTS) will be released at the end of May
Symfony 2.4 will be released at the end of November 2013;
Symfony 2.5 will be released at the end of May 2014;
After the release of Symfony 2.3, backward compatibility will be kept at all cost. If it is not possible, the feature, the enhancement, or the bug fix will be scheduled for the next major version: Symfony 3.0.