I did search alot, looking for this, but could only find these early WotLK threads about vehicles not giving fall damage.
So if you are a rogue, above level 40 (when you get Safe Fall) + Safe Fall Glyph, you can mount up onto a Mechano-Hog or any other vehicle. When your friends hop onto your mount, they will always recieve the same amount of fall damage.
Let's say jumping 100 yards down takes 90% of your HP unless your a rogue.
If you are a rogue WITH safe fall and Safe Fall glyph, you will only loose 30% HP.
While mounted, everyone in your mount receieve the "Safe Fall" Bonus as long as they are inside your mount.
So:
1. Be Rogue above lvl 40, get Safe Fall, get Safe Fall glyph, get vehicle mount
2. Mount up and your friend(s) hop onto your mount
3. Jump a jump that would kill you if you weren't a rogue
4. Survive
5. ???
6. Profit
It's not the biggest SPLOIT on WoW but gives small advantages such as in the begining of EotS, you can jump straight out and not loose any HP.