Good news for SMP server XP farms
for now we can kill the skeleton and Zombies with hit them on the legs :)

Here is a Really good Video about Building XP Farm

minecraft xp farm for Skeleton and Zombies, Zombies need 2 hit to kill. as you can see here, we can build the xp farm a lot different ways

Good luck with your Build
if you have any questions just ask me :)