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:
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:
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:
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:
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 accoridng to the developers. I will assume that there's text somewhere indicating that a character who falls that way is rendered Prone.)
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. The above quote from the
What are the effects of being Prone?
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.