Using some less-than-orthodox methods, I found my way under Northrend, and from there, I was able to fly into one of the most likely candidates for the entrance portal to the future Icecrown Citadel raid: Angrathar, the Wrath Gate. Posted below are the screenshots I took of what I found. Bear in mind that this was done on a live server with the current patch (3.1.3):
Picture 1:
Here in Picture 1, I am flying under Angrathar, looking out towards Dragonblight. You can see Alexstrasza and the destruction from this vantage. At the very top of the picture, you can see the bottom of the "teeth" of Angrathar's gate. (Also, take a moment to note where my mini-map says I am).
Picture 2:
In Picture 2, you can clearly see the "teeth" of the Wrath Gate behind me while I stand inside the cavern behind it. I'm sure they will only open once you have completed some sort of attunement quest.
Picture 3:
In Picture 3, you can see me flying just under Angrathar once again. There, you see an interesting black space in the wall of Angrathar. It looks suspiciously like a place saver for a portal from here, so I decided I needed a closer look. (Note the mini-map once more.)
Picture 4:
When I flew back into the cavern behind Angrathar's door, I took a picture of this black wall. From this angle, it does very much look like a portal space, and considering that the Lich King himself exited from this very gate in the Dragonblight cut-scene, I am almost positive this is where Blizzard plans to implement the entrance to Icecrown Citadel.
__________________________________________________
From what I have seen from where I took these pictures and from the quest-chain that led here, I can say with quite a bit of certainty this is where the entrance portal to Icecrown Citadel will be. Now I know there will be people who will say "lol that wuz obvious newayz lololol," but even so, such speculations still needed proof, and here it is.
I hope you enjoyed your sneak peek!