The Bukkit team has temporally disabled that in craftbukkit.
These are the build issue from EvilSeph's post on bukkit.org
Known Issues:
Torch flame graphical glitch resulting in a torch's flame flickering randomly on the screen. This is a Minecraft bug that can't be fixed by Bukkit.
Client displays "Unable to locate sign" debug messages under certain circumstances. This is a Minecraft client change that can't be fixed by Bukkit.
Monsters, villagers, etc. using portals (added in Minecraft 1.4) has been temporarily disabled.
For more detailed information on what is contained in this update, please see bukkit.org
