Mobs spawned with enchanted armor do not receive any benefits from said enchantments. This is subtler with naturally-spawned mobs, but mobs which steal powerful armor or are spawned with it (via custom spawner) also receive no benefit from the enchantments.
What I expected to happen was
Zombies with feather falling gear could survive high falls, and zombies with full-strength protection armor would be difficult to kill.
What actually happened was
Sorry, no dice. Just as weak as normal zombies, pretty much.
Steps to reproduce:
1. Prepare a pair of Feather Falling IV diamond boots via an anvil and an enchanted book, if you do not already have a pair.
2. Build a 24-block tall tower.
3. Drop a copy of the boots on the top of the tower. You can make copies with the Pick Block button on the item in your inventory.
4. Spawn skeletons or zombies until one picks up the boots. You should be on Hard difficulty, and the gamerule mobGriefing should be true, in order for this to work.
5. Push the mob off (do not punch it).
6. Observe that it dies upon impact.
7. Wear the boots yourself, switch to survival mode, and jump off.
8. Observe that you take roughly 5 hearts of damage. You survive.
9. Acknowledge that skeletons and zombies have 10 hearts. If the enchantment had acted on them, they should have survived as well.
After observing this with normal items, I tried items which were forced to have enchantments at excessive levels (e.g. level 10), and the armor still had no effect. I did some testing, and no, it's not just that protection enchants have inherent randomness or that there's a cap to EPF (enchantment protection factor), as the armor allowed me to survive in situations where the mobs didn't. This may be related to the fact that Respiration doesn't work on squid, but I've yet to test that on zombies.
At any rate, while the issue is subtler in normal gameplay (although it's a shame a zombie that stole your best armor is still as easy to down with a bow as any other zombie), it's a major problem for adventure maps that rely on custom mobs which are meant to actually be difficult to kill (or survive high falls, for that matter).
EDIT: Observation: The new Thorns enchantment will work on mobs. Protection, Feather Falling, and others will not. Dinnerbone has told me that all armor enchants work on mobs, but I am afraid he may just be mistaken because one armor enchant does work. Skeletons with Protection X armor can be downed in four hits of an iron sword in hard mode, just like normal skeletons. Feather Falling still fails to reduce fall damage to mobs, putting a damper on Vechs' ideas of leaping horrors. This still has not been fixed, and is even present in the most recent snapshots.
It should also be noted that weapon enchantments seem to work correctly on mobs, even skeletons who deal 1.5 hearts melee damage on hard mode regardless of what material their sword is made of.
EDIT 2: I have done more testing. Feather Falling and other Protection-class enchantments do not work on mobs; their EPF is entirely ignored. Thorns and Respiration do work on mobs, as these enchantments do not function via EPF.