Zombie Pigmen are not defined as undead in the game's code, therefore they don't receive damage from the smite enchantment.
Comments 6
I tested it real quick in 1.4.4pre: Unenchented diamond sword: 4 hits to kill 1 pigman. Smite IV: 2 hits to kill 1 pigman.
EntityPigZombie extends EntityZombie, so it should inherit that attribute. Have you actually tested this in-game? Zombie Pigmen are armored (2 points), which may explain any difference in actual damage from expected damage.