While it looked like the new 25 man on the PTR, the Sunwell Plateau, was going to be staying open after a few closures in the recent weeks, luck would have that it's not.
Daelo announced a short time ago that the instance will be locked until further notice to fix a bug in Lady Sacrolash and Grand Warlock Alythess. What this bug is exactly, we don't know. But I'm sure it has to be quite a good one for them to lock everyone out of the place.
Keep an eye out on WoW Insider. We'll let you know when the Plateau is back up and patch 2.4 is ready for more action.