I dislike the 3.0 version of shield, and it creates a number of wierdnesses (like this), but given the rules as they are, it's a reasonably valid whinge (and 3.0's Shield is so broken, I'd be inclined to rule against it by default).
Some interesting bits though..
1. Gotta make the Jump check to clear, say, at least 5' over the Mage's vertical reach (in 3.5 that would require a check of 20 or more). If you picture the spaces around the Mage as 5' cubes, you'll note that the Monk is likely traveling through an adjacent 'cube' and ought to suffer an AOO thereby...
If he grabs SERIOUS air, I'd let him add 'Falling Object' damage to his attack (in 3.5 a typical character - 101-200 lbs - would need to fall 20' feet to do an additional d6). Might thereby avoid moving through a threatened square too...
2. Attacking from above gives him the +1 to hit
3. If the Monk did it by running up, (straight line etc.) I'd let him have the +2 for charging too (it really wouldn't apply unless he were charging from straight above the Wiz, but it's interesting)
4. Now the Monk has to land. First, the Monk is entering the Mage's square. If the mage is in a position make another AOO, then he gets one. Now the square is double occupied, so I'd rule that the Monk falls prone in the square (unless, of course, he's managed to drop the Mage already)...
If the Monk had Spring Attack, I'd let him make a Tumble Check to pass through an occupied Square (DC 25) and finish his move wherever he's got the movement to reach.
Make it REALLY interesting and take the previous round's Standard Action to 'Feint' - which would deny the Mage's Dex and Dodge bonusses as well...
Now I'm imagining what, say, Stone Giant Monks could do to a party using this sort of technique...(+1d6 for ea. 200 lbs for ea. 10' increment...)
Amal