[sblock=Reach Weapons]I have had this come up multiple times today. Here in PF and elsewhere in 3.5ed. While I have found the answer in 3.5ed, I have not confirmed it in PF, but I imagine the ruling would be the same.
Reach weapons determine cover like ranged weapons. However, like melee weapons, there are not affected by soft cover. Thus, an attacker wielding a pole arm against a defender who is standing behind another creature (whether the attacker’s or the defender’s ally) is not hindered from making an Attack of Opportunity against the defender, nor does the defender gain +4 AC.
This would make a difference in The Duke's attack.[/sblock]