Areal attacks?

[Kaeso]

I have a quick question about areal combat, because the rules are a bit vague about it. Let's say that a creature dives down from the air to attack an earthbound enemy with his melee attack, then they just enter combat (assuming they're the same size and have no reach weapons) and all is fine.

However, what happens if one has fly-by attack? If an areal creature like a harpy dives down from the air, smacks a human in the face with her club and retreats back into the air, does the human get an Attack of Opportunity? Usually you get an attack of opportunity if somebody tries to disengage from combat, does the same apply to areal combat with fly-by attack or is this a different affair?

[Krobar]

A creature doing a flyby attack provokes an attack of opportunity, unless they have the feat Improved Flyby Attack.

http://www.d20srd.org/srd/epic/feats.htm#improvedFlybyAttack

[Adamosaur]

The above poster answered your question, but a common way for a melee based party to deal with a creature of this type is to ready attack. So when the flying creature swoops by to attack and flyby, the fighter (or melee characters) get a solid swing off as the flying creature comes within range.
Otherwise, adequate ranged attacks can take down such a creature (let alone a few useful spells from high enough level castors).

[Greenish]

The AoO obviously relies on the guy on the ground having equal or longer reach than the flyer. Whether the readied attack works vs. longer reach is probably down to DM interpretation ("if it reaches you, you reach it" vs. "you can't hit something outside your reach").

[BowStreetRunner]

The most important concept to keep in mind with regard to aerial combat is that the 5 foot squares used to keep track of movement in the game are actually 5 foot cubes. You actually threaten not just the squares around you, but also the ones above you and below you. Ceilings and floors work the same as walls in this regard, creating cover when present. Movement from cube to cube works essentially the same up and down as it does side to side.

[Adamosaur]

Indeed readied attacks versus a flying creature with reach would be DM discretion.
In my opinion, if the creature is using natural attacks (talons or claws) then the fighter would be striking at those claws. Unless the flying creature is using a sword or a spiked chain (you'd hit the chain instead of the wielder, in this case you'd hit the talon/wing/tail which is the wielder/creature) then I'd personally rule it do-able.