My Item class has it. Just look at the type hierarchy in Eclipse and it will list all the methods.
Here is the actual code from the Item class:
* Sets the unlocalized name of this item to the string passed as the parameter, prefixed by "item."
public Item setUnlocalizedName(String unlocalizedName)
this.unlocalizedName = unlocalizedName;
Maybe your class path is wrong and you're looking at the wrong Item class or something?