When a character gets up from prone, when does the
attack of opportunity take place? When he is still prone?
When he is standing? Can the attacker choose when to
attack? In one case, the attacker can get a +4 bonus to hit.
In the other, he can make another trip attack. All attacks
of opportunity happen before the actions that trigger them (see
Chapter 8 in the PH). When you make an attack of opportunity
against someone who’s getting up, your target is effectively
prone, and therefore cannot be tripped. You could ready an
action to trip a prone foe after he gets up, however.