Apologies to all who posted between my previous post and this one- I'm preparing my house for an influx of relatives fleeing Ivan, so I'm not taking the time to read this stuff right now.
I'm not ignoring those previous posts, however, and will return to them as soon as possible.
However, I found this to be worth posting in my few moments online tonight.
A little research on my part has turned up an error that we have largely ignored. We mostly assumed that the 3x Designers made all or almost all undead irrevocably evil. They did not. Besides the Osiris example brought up before, and the Deathless of BoED/Eberron, I turned this up-
Undead Alignments by book:
Monster Manual
Mummy-"Usually Lawful Evil"
Monster Manual 2
Banshee, Deathbringer- "Usually neutral evil"
Corpse Gatherer - "Usually Neutral"
Gravecrawler - "Always Neutral"
Jahi-"Usually Lawful Evil"
Ragewind- "Usually Chaotic Evil"
Death Knight "Same as character (always evil)"
Magic of Faerun
Crypt Spawn- "Usually Evil"
Dieties and Demigods
Osiris-"Lawful Good"
Greater Mummy-"Any"
As you can see, some of these undead are given alignments that can vary. One, Osiris, is a unique being, so he can be discounted. However, note that the Crypt Spawn is only "Usually Evil," the Corpse Gatherer is "Usually Neutral" and the Greater Mummy can be "Any" alignment.
(The Death Knight is presented to show that when the designers want to say that something is always evil, they do.)
So, in a sense, this thread's predicate quesion is incorrect: "Undead = Evil" is not the case in 3xEd. There is merely a presumption of evil, and DMs are given the option to control when.
The question THEN becomes why are some evil and not others- why is the equation not true? As I have stated before in previous posts, I believe that is because of the will of the creature becoming undead.