According to the Rule Compendium, "A winged creature can be tripped, and if it is, it falls as if it didn’t maintain its minimum forward speed." (P. 145) But if the target has good or perfect maneuverability it has no minimum forward speed. I'm inclined to interpret it as falling anyway, since the wording doesn't seem to imply you actually need to fall below the MFS, just you fall "as if" you had. Has this ever been clarified?
Ultimately this comes down to triggering a condition, vs the results of that condition.
First, here's the Minimum Forward Speed rules:
If a flying creature fails to maintain its minimum forward speed, it must land at the end of its movement. If it is too high above the ground to land, it falls straight down, descending 150 feet in the first round of falling. If this distance brings it to the ground, it takes falling damage. If the fall doesn’t bring the creature to the ground, it must spend its next turn recovering from the stall. It must succeed on a DC 20 Reflex save to recover. Otherwise it falls another 300 feet. If it hits the ground, it takes falling damage. Otherwise, it has another chance to recover on its next turn.
Good and Perfect maneuverability (and anybody with the Hover feat) don't have a Minimum Forward Speed, and as such can't meet the triggering condition here.
Rules Compendium (p. 145) says this about tripping a flying defender:
A winged creature can be tripped, and if it is, it falls as if it didn’t maintain its minimum forward speed.
This is a new triggering condition. If you get tripped, you treat it as if you didn't maintain your Minimum Forward Speed. Thus, reading the first rule again, the part where it says "If a flying creature fails to maintain it's minimum forward speed" is now true. You did not maintain it (even though you don't normally have one), because the trip rule says explicitly that you failed to maintain it due to being tripped.
Given that, what happens if you get tripped that you fall per the rules on minimum forward speed.
Two things worth noting:
- The rule says a winged creature can be tripped. By a strict reading of that, anything that flies without wings (mostly things with Supernatural flight rather than Extraordinary flight) are immune to being tripped.
- The Minimum Forward Speed rules say that you have to land at the end of your movement, or fall and take damage. Trips happen on someone elses turn, so you have no movement and can't land. The tripping a flying defender rule says that you fall, so the strict reading is that you don't get to try to land and immediately fall 150 feet (and maybe take falling damage). If you don't hit the ground, you can use your next turn trying to recover, as described in the rule.
I would look at "how" that creature flies. If it uses some sort of magic-powered levitation, I would not say that it "falls" at all. Sure, it can stumble out of control for a few feet, but it would not fall. Beeing Winged don´t means muscle-powered fligth.
On the other hand, if the creature uses some muscle-powered way to fly (like wings with no magic support), I can see it losing momentum and going to the ground. The rules, on that prospect, is not as clear as they should IMHO. As I see, that rule should apply only to this case.
From the Link that Colin D provided:
Creatures that fly without wings (and any creature with perfect maneuverability) can't be tripped while flying. If the creature is still in the air after stalling, it must succeed on a DC 20 Reflex save to recover and resume flying. Otherwise it falls another 300 feet. If it hits the ground, it lands prone and takes falling damage.
So... It gets pretty clear.
My argument falls under the idea that "Prone" as a result of being tripped is more about a condition than it is about a physical description. (On a side note, based on the quote you have above, a tripped winged creature isn't necessarily prone unless I'm missing something about falling due to Minimum Forward Speed, but that can't be helped since I don't have that book and the Rules Compendium may not be considered valid errata according to the developers. I will assume that there's text somewhere indicating that a character who falls that way is rendered Prone.)
A tripped character is prone. Standing up is a move action.
Unless the flying creature specifically has a note saying that the Prone condition doesn't apply with Perfect Maneuverability, I'd say that it applies.
What are the effects of being Prone?
The character is on the ground. An attacker who is prone has a -4 penalty on melee attack rolls and cannot use a ranged weapon (except for a crossbow). A defender who is prone gains a +4 bonus to Armor Class against ranged attacks, but takes a -4 penalty to AC against melee attacks.
Standing up is a move-equivalent action that provokes an attack of opportunity.
That's the mechanical aspect of it. The big kicker is that first line of "The character is on the ground". Strict interpretation of the rules, that tripped flying person is on the ground. Kind of silly if you're fighting a levitating opponent who's two miles up in the atmosphere, so one could argue that the first part is just supposed to be a suggestion of what Prone is supposed to emulate, getting knocked down. Now, what's the flying version of for someone with perfect maneuverability?
In my mind, that's a barely controlled tumble. It's the guy levitating who's been thrown off balance and has gone into a bit of a spin. He can't coordinate his hits as well. He's exposing more surface area for a hand weapon to go through, but a ranged attack has a decent chance of missing due to his erratic movement. He can't move as fast because he's not entirely in control and, until he takes his Tumble check or spends a Move action to right himself, that will continue (and taking that Move action means essentially stopping himself and making a target for the AoO as he straightens himself out).
I will admit that this interpretation came more as a way of resolving trip attacks in a d20 superhero RPG, Mutants and Masterminds, but it works with Rules as Written in any d20 system that defines Prone in this manner.
I accept my downvote for being a bit fuzzy with that first line of "You are on the ground", but I think it's just one of those cases where common sense trumps it.