Since there a plenty of examples of Undead with weaknesses -- Vampire's getting staked through the heart, Living Dead getting shot through the head -- I see no problem in allowing a Rogue to Sneak Attack the Undead with the proper Feat. Supernatural Blow, from Masters of the Wild allows a Ranger to use his Favored Enemy ability against the Undead (or Constructs) and score Critical Hits. I don't see why it shouldn't be used by Rogues as well. Perhaps you might require the Foe Hunter Feat first -- and you might make each kind of Undead a different Foe.