Let us begin and take a look how we shall get under Northrend, shall we?
Well, you first need to take your flying mount with you, get to around Shadow vault and go to the highest point of the Ulfar's chamber or whatever.
http://img372.imageshack.us/img372/2821/wowscrnshot112408175002ag3.jpg
There!
Then go through the hole and fly through the other hole under you.
Now, face forward. Fly to the last snow you can see on your side, DO NOT GO FURTHER! Yet.
Now, fly fast and swift across to the other side! Do not, i repeat, do not stop or go under by "mistake", then you will get dismounted and will need to restart.
Now you will be here:
http://img372.imageshack.us/img372/2627/wowscrnshot112408175748uk8.jpg
FLying mount on, fly through the hole you see on your left.
http://img372.imageshack.us/img372/3815/wowscrnshot112408175812un2.jpg
Taa-daaa!
by Kromgol