Snapshot 13w18a, Snapshot 13w18b
I have set the world's gamerule doMobSpawning to false.
Attacking spawned Zombies or Zombie pigmen in the overworld causes zombies to spawn, using the new mechanic, and attack me.
This is true for both creative and survival.
MC-14722 Social Zombies not affected by /gamerule doMobSpawning