Hypersmurf's got it right about the Tumble check while flying as long as its part of normal movement. However, you may have forgotten to check the maneuverability guidelines. That half-dragon should have had average or worse maneuverability and thus had to move half his normal fly speed going forward or he stalls and falls 150' (or to the ground whichever is closer) at the end of his move. Tumbling reduces his rate by half automatically (unless he did the quick tumble and took a -10 to his check), and if he made any turns or gained/lost elevation, that would reduce his move further and likely would result in him stalling and falling either in front of the scorpion or at best on its back at the feet of the lich. And since falling is not flying, the scorpion likely could have got an AoO and maybe caught him in a pincer or something.
I disagree with Hypersmurf's "Crawl speed" thing. I checked through the MM and didn't notice crawling listed as any type of movement, or in the PHB or DMG. If he wants to institute a "crawl speed" for his games though, that's his choice. Crawling is not a special type of movement or one that requires its own distinction (as opposed to flying, burrowing, etc.), it is just ground movement done in a prone position. You're still moving across the ground, just on your hands and knees. Sea creatures have a swim speed, but they don't have different modes listed for doing the 'doggie paddle'. Flying creatures aren't given a glide speed either, so no reason that a crawl speed would be listed for those on the ground. Its common sense, but that's something that's often overlooked by people with the attitude 'if its not in the books specifically, it can't be done or it's not true D&D'.
Now since Tumble involves rolls, flips, etc., I don't see anything in the Tumble description that says the character cannot roll away from an adjacent target or flip to his feet as part of his movement (that would not provoke an AoO with a successful check). In fact, it is in the Tumble description that the character can ROLL, FLIP, SOMMERSAULT, or whatever through threatened areas without taking an AoO (on a successful check) and if you don't think Tumbling can be started from a prone position then you should go see some Chinese acrobats or Cirque Du Soliel or something like that. If your base move is 30', then you could Tumble up to 15' away from an adjacent enemy without taking the AoO. And if you can go from a standing position to a prone rolling one with this skill, then you can go from a prone rolling one to standing as part of the Tumble.
Think of how many martial arts movies you've seen where one person is rolling away from an enemy or flips up to their feet as soon as they get knocked back. If you want a couple of examples, take a look at the Mortal Kombat movie (the first one). In one of the first fights, Liu Kang fights a monk and at one time or another both are prone and flip up to a standing position instantly. In another fight, Liu Kang knocks Sub Zero down but the ninja still kicks him in the face as he's falling and then flips back up to his feet in the same motion! Not saying that movie runs by D&D rules, just that it provides examples regarding Tumbling and getting to your feet, moving through threatened areas, etc.
