Seeing as the price is exactly 1000gp greater, for all types of adamantine, regardless of bonus, than similar magical weapons, then we can assume that the 1000 gp is meant to account for the indispelability/inflexibility/extra hardness of the adamantine. If adamantine does not overcome DR, then it is indeed very weak, and yet somehow it costs more than an "equivalent" magical item.
If you want to argue on strict basis of rules, then adamantine does bypass DR because it is an enhancement bonus, and enhancement bonuses bypass DR. If you want to argue on the basis of intent, then adamantine bypasses DR because otherwise it would be dreadfully underpowered for it's incredible price and rarity.
Concretebuddha, if you check the SRD under special abilities, you will see nothing that says anything like "nonmagical weapons do not bypass DR." In fact, if you read carefully, you will even see that it does not limit the bypassing of DR strictly to nonmagical weapons, and explicitly says that "a certain type of weapon—usually a magic weapon" can overcome DR. And indeed, the word "nonmagical" isn't even mentioned anywhere near the DR section, and only mentioned twice on the page (under the "extraordinary abilities" and "incorporeality" sections).
The FAQ is indeed very specific in referring to a weapon's magical enhancement bonus bypassing DR. But I refer you to "a square is always a rectangle, but a rectangle is not always a square"..in other words, a magical weapon (a square) is able to bypass DR (is a rectangle), but that does not mean that a nonmagical weapon with an enhancement bonus (a non-square rectangle) is not also able to bypass DR (is not also a rectangle).
