Xarlen, I wish you wouldn't keep using the same reasoning that has already been refuted...
Here's a sum up of my responses to Xarlen's opinions, while we were having this debate in another thread:
"Special material" has nothing to do with adamantine. "Special material" is meant to refer to silver, wood, mithral, and other nonmagical materials which do not grant enhancement bonuses but which can bypass certain types of DR.
Adamantine gives a natural enhancement bonus to the weapon it is made out of. I could not see how this could be more clear, until IanB came along and pointed out the description of the adamantine battleaxe in the DMG. This proves, beyond a shadow of a doubt, that adamantine bypasses DR...since DR requires an "enhancement bonus" to be bypassed, and adamantine gives something an "enhancement bonus". It doesn't get any simpler than that.
Also, the argument about "magical attacks bypassing DR" is irrelevant, because magical attacks are simply one of the exceptions to DR. There is nothing that says that DR much be bypassed by magical attacks, and indeed it even says in the description of DR that there are many cases in which it gets bypassed by completely mundane attacks (it uses the phrase "mundane fire" as an example of something that bypasses DR). So no, things do not have to be "magical" to bypass DR.
And, once again, the argument that "masterwork weapons not bypassing DR is a contradiction" is also false, because masterwork weapons do not give an enhancement bonus, they give an unnamed bonus that is explicitly stated not to stack with enhancement bonuses. Since masterwork weapons do not give enhancement bonuses, then they do not bypass DR. It's very simple.
And for some replies to comments made in this thread:
DR is always a supernatural ability unless denoted otherwise, and the only cases in which it isn't that I've been able to find are in the cases of werewolves and forsakers (and the aforementioned case of the barbarian...this also applies to other classes that get x/- DR, but the - is irrelevant when we're talking about overcoming DR, so I'm not really going to discuss barbarians or anyone else who gets that type of DR). In the case of a werewolf it's easy to overcome even in an antimagic field, because it's silver and silver doesn't get negated by antimagic fields. With forsakers it can be above +2, but other forsakers can get around that DR and it's really not a massive amount compared to the level at which you get it anyway.
Also, if the "adamantine doesn't bypass DR" side is basing it's argument on a sean k reynolds rant, then I don't really think that adds any credibility at all to that argument...I mean, c'mon, the guy argues that invisibility would cause people to explode.
And now, for the balance aspect of it all:
Adamantine costs more than equivalent magical enchantments, is not as flexible, and is set to a specific amount depending on what type of item that is being used. It also cannot be further enchanted, and the enchantments on it can't be anything other than a simple enhancement bonus. All of this is in exchange for it not going away inside antimagic fields (which almost never is encountered, and even if it did go away it's a very minimal effect since DR goes away inside antimagic fields anyway), not being able to be dispelled (which people don't try to do very often anyway, there's far more effective things to do than try to dispel someone's wimpy +1 or +2 sword), and having slightly higher hardness than steel (which isn't even that huge a difference, since magical swords get bonuses to their hardness and such anyway). Also, adamantine weapons and armor do not get the advantages of being masterwork, so the armor doesn't lose any of it's armor check penalty and the weapon wouldn't retain a +1 to hit if it were dispelled. So the net bonus of having an adamantine weapon is slightly higher hardness, +1 damage (or +1 hit, +2 damage) in an antimagic field, and that it doesn't get dispelled. And these minimal bonuses are in exchange for a higher cost, a LOT less flexibility, and supposedly the lack of an ability to overcome DR? Without being able to overcome DR, adamantine is too weak, and with the ability to overcome DR it is just about right.