I apologize if this is a repost but I couldn't find anything similar.
Now then, if you head up to the Emblem trading area of the Alliance Quarter of Dalaran you'll see 3 windows, go over to the one on the right as seen below:
Run directly into the area between the right part of the window frame and the tree, you should fall through the world and land on either the roof of the Inn in Underbelly or in between the Inn and the Bank, this is where you want to be.
Just run to the Left and you'll be behind the bank, to get out again just run through the grate as it has no collision on this side.
Edit: Bugger, should this have been in the exploration section?