One problem is that's it's not clearly defined exactly how Smite Evil is used.
It's a [Su] ability; [Su] abilities are standard actions unless specified otherwise.
The description for Smite Evil states "a paladin may attempt to smite evil with one normal melee attack".
This could mean:
a/ As a standard action ([Su] ability), the Paladin may activate his Smite Evil power; the bonuses will apply to his next melee attack (like DotF Divine Might, or a Feint, for example).
b/ As a standard action ([Su] ability), the Paladin may Smite Evil, which allows him to make a single melee attack (like Core Rules Sunder).
c/ The Paladin may declare (as a non-action) any melee attack to be a Smite, whether it be an attack action, charge action, full attack action, or AoO (like FAQ Sunder).
If you use c/, then the Paladin can Smite on an AoO. If you use a/, and the Paladin activated his Smite power with a standard action in the previous round, then AoO (as his next melee attack) will be a Smite. If you use b/, he cannot Smite on an AoO at all.
-Hyp.