Seems reasonable that dropping from above or jumping down on someone could cause more damage. However, if you are attacked while you are IN the air, it would seem reasonable that the "leaping" or dropping person could also stand to loose more than he/she gains on the attack bonus.

I would think that being hit mid-air could also result in falling "flat" on your face (similar to rolling a 1 in D&D) on the attack.

Hmm, if you fell on a person wielding a pole arm that planted it on the ground in response... that could be quite humorous! Attacker on a skewer?!

Taxman

"Any group is weaker than a man alone unless they are perfectly trained to work together."
- Robert A. Heinlein, Starship Troopers