There are two instances that are seen in the world, but not accessable.
* The Black Temple is located on the eastern edge of Shadowmoon Valley. It was formerly known as the Temple of Karabor and served as the inner sanctum of the draenei Prophet Velen. When the Horde began massacring the draenei, the Shadow Council under Gul'dan took possession of this as their citadel, named the Black Temple. After Gul'dan crossed into Azeroth, it became known as Fortress Shadowmoon and was ruled by Ner'zhul and his clan. Now it is the citadel of the Lord of Outland - first the Pit Lord Magtheridon, and then the half-demon night elf who deposed him...Illidan Stormrage.
The Black Temple will be opened in Patch 2.1.0.
* Zul'Aman is the capital of the Amani forest trolls that reside in Quel'Thalas. Little is known of what the instance itself will be, other than the fact that it will be a raid instance of some kind - whether 10-man or 25 remains unclear. It is located in the Ghostlands, the southernmost of the two areas controlled by the blood elves.
(post courtesy of Wikipedia)