*Note, I have only tested this on Humans and Worgen in Human form. It definitely works on Humans, and Worgen sometimes.*
The exploit requires [Black-Ice] (Ring from the quest in ZG that visually appears on your characters hand.
The trick is simple, unsheath your weapon, then equip the ring in the upper ringslot and attempt to shealth your weapon once. You will notice your character's hand will make the motion of sheathing your weapon, but it will remain in your hand. Now simply mount.
This is neat because regardless of what you do the weapon will remain in your hand. This can make for some epic poses via a well timed Deepstone Potion.
Try out:
/point
/cheer
/wave
/no
Side Note: While the appearances is server-side, it seems that it only works on players who were present when you performed the trick and attempted to sheath your weapon in there line of sight/proximity. Newcomers will not see the weapon in your hand, but on your side/back as per normal.