You can disenchant any weapon or armor that is above common quality (i.e. green, blue, purple, or orange). You cannot disenchant containers, trade goods, reagents, recipes, or any other items of the like, even if they are green, blue, or otherwise.
edit: An exception to this rule is anything you make using the Enchanting skill itself, i.e. wands. You can't make a wand and then disenchant it for components again.
Edited, Tue Apr 4 04:36:04 2006 by GDSlemons