I wasnt able to find this way somewhere, if its a report i can confirm its works in 4.0.1 !
First of all, run to the Voilet hold..
Walk under the bridge like here (You will need water walk from a shammy, dk or buy a potion)
Go up to the wall and use your ground mount, just when your about to ride up, jump forward, you should end like this.
Now walk to the right on the water to the edge.
Mount up on the mount you want to "show off" in Dalaran and take off !
It can take some practice not to get too high on the ground, witch means you will get dismount ! But its easy.