I have to agree with Eric on a couple of points. Evil, presented in the core D&D books, isn't evil at all, its very cartoony. That is, the villians are absolute- completely and utterly evil. Many times, in published modules, sourcebooks, etc, villians are given no more motivation than "they are evil, so they want to conquer the world and inflict suffering", or "they are following the will of their god/demon and do as he instructs", or "they have lost a loved one/power/feel they have been wronged by the world, so they have cooked up this idea for vengeance". IMO, this isn't an evil motivation, its more of a cop-out so that we, as gamers, don't have to really consider the implications of true evil. This works great for a fairly lighthearted campaign where alignment is absolute, the PCs gallavant around righting wrongs, smiting the infidels.
What I am about to say may sound extreme, but I am not saying it intending to start a flame war, so bear with me. What most of us would consider truly vile evil is not something most "normal" people can come up with on their own. Sure, everyone is capable of doing evil, whether its betraying a friend for personal gain, sullying someone's reputation out of spite, or even killing for passion (not that I have ever done any of those things or condone them). What we would cal truly sick, deranged evil only really comes from the mind of someone who is mentally ill. I have extensive training in biology and psychology, and if you have ever been around someone who is truly mentall ill, its a rude awakening. Without going into the eitology or progression of mental illnesses, its enough to say that they lack the inhibitions, values, and morals that allow normal society to function (relatively) smoothly. I don't think any of us would argue that Jeffrey Dahmer is much more evil than someone like Cyric in the FR or Iuz in Greyhawk (at least as they are presented in books).
Having said that, to me, truly evil and memorable villains are those who are mentally ill in some way. Yes, they might be powerful, but they have at their core flaws and weaknesses that prevent them from being a part of normal society. They are often able to pass themselves off as normal for a while, and because of that they are all the more terrifying and abhorrent. If these disturbed people are that frightening to us, imagine how completely alien and vile something like a demon should be. In contrast, the enemy who hungers for power, money, etc and does evil in pursuit of this, is little more than a thug rather than a true villain (thats not to say that villians might not want money, power, etc either).
To me, the recent "mature" section in Dragon was laughable- it was still cartoony and went for the gross-out factor, without really provoking much thought into the nature of evil. In retrospect, there was nothing mature about it at all. I hope the BoVD does deal with, in a truly mature manner, the nature of evil, and implications of those actions rather than throw lots of spells, feats, and stats at us. IMO, it is hard to make a truly memorable and horrific villian without dealing with some aspects of human nature that make us uncomfortable. Hopefully, but dealing with this sort of thing in an adult manner, it will encourage people to think about the consequences of their actions (and their character's actions) and how they affect others.
Religious scholars have contemplated the nature of evil for thousands of years, so I don't see how one game book dealing with evil is any big deal at all. I really don't think the BoVD will make any difference in most people's perception of RPGs and D&D in particular- its just not a controversial topic anymore. IMO, the people who are screaming about how wrong it is to have things like the BoVD have no ground to stand on. If you don't like it, don't read it, and don't use it- your game won't suffer any without it.
I know a lot of people may be uncomfortable with what I have said here, and if I have offended anyone's sensibilities, I apologize. I was just hoping to provoke some thought and mature discussion on the nature of evil in D&D.