I was in Creative Multiplay, with doMobSpawning false (Because I was working on dark area, and I hate bats spawning.) I put trip-wire triggered alarm system so I can use it when intruder come into private area, it triggers Command Block.
I was working on map for days, and suddenly, that alarm triggered.
Since doMobSpawning was false, and no one was there without me, and I was working on different area, nothing could trigger that alarm.
I shit my pant afterward and checked area with MCEdit. I found it was pigZombie, which was Zombie Pigman, spawned from near Nether Portal.
So yes. Zombie Pigman from Nether Portal still spawns even if doMobSpawning is false.
Although this was Creative Multiplay, I'm pretty sure this will also occur for Survival and Adventure.
Sorry for no screenshot or video, since I never expected this to happen, I couldn't take a shot.
Steps to reproduce is simple, just turn off doMobSpawning, create a portal, and wait until game spawns Zombie Pigman. However this might take very long time. As I never encounted this problem until now.