Bukkit

It is the default world.txt file :

#This is the UltraBuild config file for worlds.
#Define your worlds in which UltraBuild are enabled below:
world=true
AnotherWorldNameHere=true

For enable in a world you just have to add this : "TheNameOfYourWorld = true" or change true to false for disable UltraBuild in that world. And after you have to reload/restart your server.