Since the snapshot 13w18 and the resolution of the bug MC-590 (Mobs not realizing they killed their attacker and trying to attack empty space), the zombies now stop targeting villagers as soon as a snow golem attack them.
They didn't react towards snow golems before when the were chasing a villager.
That's a big issue for mob sorter, because the only way to nicely separate them is to use :

• snow golem for the skeletons
• villagers for the zombies
• cats for the creepers