1. Must be Hunter(?)
2. Be in battle.
3. Talk with the flightmaster while being in battle.
4. Feign Death.
5. while you are Feigning death take fligtpath to anywere.
6: It now bugs up and you can steer yor Gryphon were you want
It doesnt move at the gryphon speed. But still cool moving around with a gryph I found this out in Tanaris today.
Update! You can fly under water without drowning! Tried to pass by "fatigue" but you will end up dying without even seeing that you lose hp.
You dont aggro. Other ppl can see you and your mount.