If you drop onto an opponent from above and try to attack them, could you treat that attack as if you had set against charge and they were moving toward you at your fall speed? It may sound odd but relatively speaking it could make sense since from your perspective the ground and the target are moving toward you at the speed you are falling, so you might be able to invoke the rules for when that normally happens provided you have the actions to do so.
I would say no, both by RAW and by interpretation. By interpretation, part of the point of setting against the charge is usually using the opponent's momentum against them by forcing them to run into a stationary piercing weapon, braced against something equally stationary, usually the ground or a foot. When you're falling you don't get that benefit. By RAW, there's nothing stopping you from readying against a charge but the person standing still isn't charging, because a charge requires at least 10 feet of movement.
If you're looking to design a character based around this concept, I would highly recommend the Leap Attack feat from Complete Adventurer. This feat allows you to double the damage of a power attack if you jump more than 10 feet horizontally before the attack. I think it's an easy house-rule to say the feat would also apply to vertical movement as well. The link KRyan posted has some other pretty great suggestions as well.
That said, I would rule that the attacker is charging, and therefore the defender would be able to set against the charge. This I think makes sense, as the whole point of readying against the charge is to set a piercing weapon (such as a spear or lance) against a charging opponent. What is a charge? Well, charging usually involves rushing directly towards a target moving faster than normal combat speed and somewhat more recklessly. I would say that a falling attack certainly meets both of those criteria.
I would therefore rule that falling attack could be considered a charge, which you could then therefore ready an attack against.
Setting against a Charge is a specific mechanical option that you can take on your turn. When you do so, you prepare yourself for someone to Charge you; if they do, you get to attack them and deal double damage.
Unless someone Charges you, that preparation is wasted.
A Charge is also a specific mechanical option. The charger must explicitly choose to Charge; simply moving forward in a straight line and attacking once would not be a Charge, the attacker would receive neither the +2 bonus to attack nor the −2 penalty to AC, and would not be able to move double his move speed. He would also not trigger the Readied Attack that was set against a Charge. There is no way to Charge without meaning to do so.
You can set against a Charge, by the rules-as-written, while falling. Ask your DM if he’ll let you; the rules never actually mention the ground or being stationary, but it seems likely that it would be hard, if not impossible, to perform the real-life action that the mechanical option models while falling. Whether or not a DM will allow it depends on a variety of factors like play-style, balance, goals, and so on.
However, even when allowed, this does nothing on your turn. Even if you fall into another creature, you would not get an attack against that creature (since you’ve already used your Standard Action setting against a Charge, and that creature has certainly not used a full-round action to perform a Charge). If, during the next round, someone Charged you, then (and only then) would you get the attack.
You can instead choose to attack at the end of the fall. You could choose a regular attack (a Standard Action), or you could argue that the fall itself is a Charge (you take −2 to AC but get +2 to Attack, can move twice as far which doesn’t really matter while falling, and may receive other benefits depending on your equipment, feats, and class features). The “Are there rules for dropping on an enemy as an attack?” question also details a number of ways to weaponize falling.
Look at the Battle Jump feat in the Unapproachable East. It let's you treat a falling attack as a charge and if you hit it will double the damage or let you make a free trip attempt. You are treated as one size category larger if you trip or grapple.
Thus, someone using this would be charging. The target could set against a charge in this case, since the fall is now a charge attack.