From the "Rules of the Game " article:
Trip
Most creature using wings or other appendages to fly can be tripped. Incorporeal creatures with perfect maneuverability, and creatures that don't rely on their limbs to fly cannot be tripped when in flight.
Resolving the Trip Attempt: The attacker makes a Strength check. The defender can oppose the attempt with a Strength check or a Dexterity check. Each creature gets a bonus based on its maneuverability rating, as follows: perfect +12, good maneuverability +8, average +4, poor +0, clumsy -4.
Stability bonuses do not apply in aerial overruns.
Trip Results: A successful trip forces the defender to stall (even if the tripped creature doesn't have a minimum forward speed) rather than knocking the defender prone.