If you're using consolekit, gdm can arrange that when you "migrate" to an existing session, it will automatically unlock for you as well.

The code to implement this in gdm 2.19.4, however, fails to send this notification when the greeter (into which the user has just entered their login details) is not TYPE_FLEXI.  This is incorrect; this should be done for all greeters when migration is appropriate.

Otherwise users will sometimes get a spurious gnome-screensaver lock: this will happen when they use the greeter which appears when the first user to log in (who used the :0 server which is not TYPE_FLEXI) logs out.

I will shortly attach a patch to fix this problem.  The patch de-duplicates some of the migration code, at the expense of duplicating the if (...TYPE_FLEXI) test.  This seemed appropriate as the migration code was expanding.  An alternative would of course be to duplicate the #if...unlock_ck_session...#endif.

I have tested this patch on Ubuntu with gdm-2.19.4-0ubuntu2.