A1077 uncertain
From the fact that you can force a spellcaster to make a concentration check or lose his spell, it seems as though the AoO interrupts the action that triggers it. Though there is some feat in Draconomicon, forget its name, which triggers when someone who's within a monster's reach tries to get closer, and it's written as you pushing them back to the limit of your reach, implying that they've already succeeded in moving. Still, that might be a special case, so it seems as though the general rule is that you interrupt before the triggering action completes. Which would seem to indicate that you'd be attacking their prone AC. Better get Curmudgeon to double-check this one.