"You will give me $1,000 if I live in Boston."
I don't live in Boston, but the "relevant portion to me is the first half of the sentence." Do you owe me $1,000?
Is overcoming DR X / admantine and ignoring up to 20 points of hardness "a class ability that depends on armor use"?
No, it isn't.
Accrordingly, Adamantine Body doesn't let you act as if you were wearing an adamantine gauntlet - attacking with which, by the way, is an unarmed strike, not a slam attack.
Loki said:
It is, in almost all ways, adamantine armor bolted on you.
It's funny you should say that, because that is exactly what Adamantine Body is not - otherwise, why can't a warforged just bolt it on later? Why does it cost a feat that can only be taken at 1st-level?
No, Adamantine Body represents a "heavy trooper" version of the basic warforged model, one made with more adamantine in its frame, with additional reinforcement, than a normal warforged. It can only be taken at 1st-level because, after that, the warforged has already been made. Bolting adamantine plates on it is just giving it a suit of armor - something that warforged cannot wear.
For what it's worth, Keith Baker says you're wrong, too:
KB said:
Q If a Warforged took the Adamantine Body Feat, is it ok to treat its Slam attack as adamantine for purpose of overcoming damage reduction?
A I wouldn't, personally. The Adamantine Body is very deceptive in its current name & description. Really, it implies a more heavily armored construction with some elements of adamantine-infused material, but I have never seen it as plates of solid adamantine attached to the exterior of the body. So I don't feel that it would be significant enough to add this – get an adamantine battlefist.
And ...
KB said:
The armored body feats simply aren't described clearly. An adamantine body warforged is not simply supposed to be a standard warforged with some plates hammered on; if that was the case, why couldn't you add them later in life? These feats are supposed to reflect the fundamental construction of the warforged. A fighter with the Adamantine Body has more metal (both adamantine and steel) in his body. His joints and weak points are heavily reinforced. He is built to be able to shrug off blows that would seriously injure a composite warforged. The mithral warforged is armored, but designed for more graceful motion and fluid movement than the adamantine model. The composite is the base -- and yes, in my opinion, not very common. Warforged were made for war, and most would have one of the armor feats. The armor feats are not supposed to use as much of the rare ore as making a suit of platemail of that armor; the metal is spread throughout the body, and not as much is needed to provide the benefit (and again, in the case of Adamantine Body, the benefit also comes from the general solid construction).