Testing for the big bad raid of the Mists of Pandaria expansion is not quite ready to begin, Ion "Watcher" Hazzikostas tells us, but we're getting close. There will be a few more PTR patches to take care of some stability issues, and then Blizzard will be ready to open up the Siege of Orgrimmar to testing groups. Blizzard is going for two major schedules for SoO testing: one at 10:30 am Pacific time, aimed toward EU players, and one at 4:00 pm Pacific time, aimed toward North American players, though all regions will be open to testing during both scheduled windows.
Normal and heroic 10-player modes will be tested first before proceeding to 25-player normal and heroic modes. LFR and flex raiding will be available for PTR testing as well, and will be unlocked once a raid wing has completed testing in normal mode, with a longer time frame for testing as well.