An odd thing happened, after a fight. Had a skeleton that my wiz had created, at the end of the fight it was down to 3hp. So I thought, wth, my cleric zapped it with the wand of cure light wounds. The skeleton failed a save and perished from negative energy. Now if it had said positive energy no problem. Odd...
If your wizard is of evil alignment, to animate dead, etc and a wand of cure light wounds was used, not normally available to a wizard, then it acted as a inflict light wounds and killed the skeleton. I'm not sure if cure spells will work on undead. Normally, only clerics deal with undead, so i'm not real sure what's going on here.
No, it did get hit with positive energy. The game has a few issues with telling those two apart. We're working on it.
And just to expound upon what CtB is saying... ANY undead that has a cure xxx wounds spell (or Heal) used upon it takes damage from the spell. If it is struck by a cause xxx wounds spell (or Harm) it repairs the undead. This is as is should be based upon the 3.5 rules. Only problem is that the game engine doesn't properly report the type of energy being used.
As Noran was saying, undead are imbued by neg. energy, and any type cure spell uses pos, energy which (logically) harms undead. Just bad reporting by game engine. Use of 'inflict' spells should heal undead per 3.5 rules.