Create a new world (not sure if necessary...)
Make sure the world difficulty is set to "normal"
Spawn in some mobs using /summon or do /time set 18000
Type /difficulty 0
Mobs won't despawn!
Environment
Linux Mint 16 Petra
Cinnamon Desktop Environment
java version "1.7.0_25"
OpenJDK Runtime Environment (IcedTea 2.3.12) (7u25-2.3.12-4ubuntu3)
OpenJDK Server VM (build 23.7-b01, mixed mode)
GNU bash, Version 4.2.45(1)-release (i686-pc-linux-gnu)
Tested in a world created in 14w03b
Confirmed.