I'm going to try to clear this up and get everyone on the same page.
Here's the text about Darkwood, again, directly from the SRD:
[spoiler]Darkwood: This rare magic wood is as hard as normal wood but very light. Any wooden or mostly wooden item (such as a bow, an arrow, or a spear) made from darkwood is considered a masterwork item and weighs only half as much as a normal wooden item of that type. Items not normally made of wood or only partially of wood (such as a battleaxe or a mace) either cannot be made from darkwood or do not gain any special benefit from being made of darkwood. The armor check penalty of a darkwood shield is lessened by 2 compared to an ordinary shield of its type. To determine the price of a darkwood item, use the original weight but add 10 gp per pound to the price of a masterwork version of that item.
Darkwood has 10 hit points per inch of thickness and hardness 5.[/spoiler]
The disagreement centers around how to interpret sentences 2 and 3 of that text, the ones that start with, "Any wooden...," and "Items not...," together.
A reading of sentence 2 states that any wooden or mostly wooden item made out of darkwood is both A) masterwork and B) 1/2 the weight of an identical item made out of normal wood.
A reading of sentence 3 creates a special restriction for items not normally made out of wood or which are normally only partially made out of wood: they either A) cannot be made out of darkwood at all or B) do not get any special benefit if they are made out of darkwood.
Comparing sentence 2 to sentence 3, the former is far less open to interpretation than the latter. Sentence 3 contains many more subjective words.
In the case of this particular disagreement, the focus of the parties to that argument has been on the words "special benefit," as applied to this hypothetical Darkwood Chain Shirt.
Advocates on one side argue that the words "special benefit" include the reduced weight and that. Therefore, this side argues, because a Chain Shirt is not ordinarily made out of wood and because a 50% reduction in the weight of the item would constitute a special benefit, a Chain Shirt made out of darkwood would not weigh one-half of an ordinary Chain Shirt, but rather be the same weight as an ordinary chain shirt.
Advocates on the other side argue that the words "special benefit" and the text regarding the reduced weight are exclusive; that "special benefit" refers to anything OTHER than the weight reduction.
Analysis:
I'm a firm believer that rules interpretation is much like statutory interpretation. One of the canons of statutory interpretation is to give effect to each word or sentence in the statute, to reconcile the seeming conflict as opposed to declaring that one part of the text trumps the other part.
In light of that canon, those advocating for the "special benefit" and the weight reduction to be exclusive properties have the stronger argument. Their interpretation gives effect to the entire text.
There is additional support for this interpretation within the text itself. The text provides a description of the benefit conferred upon a shield made out of darkwood. That shield has its armor check penalty reduced by 2. This is clearly a benefit different than the weight reduction, and one which sets a darkwood shield apart from other, more mundane shields. However, since this benefit is only noted for shields, but not other items, it is unique, making it "special," and therefore more in line with the intent of the text.
Furthermore, if the interpretation of the "special benefit includes the weight reduction" advocates were to be accepted, a clear contradiction would be created. A Darkwood Chain Shirt would be an item made mostly, if not 100% out of darkwood. If the weight reduction were ruled not to apply to that chain shirt, then the clear intent of the text would be thwarted.
Conclusion:
The weight reduction is not a "special benefit" within the meaning of the text. Therefore, a Darkwood Chain Shirt has all of the benefits of an ordinary Masterwork chain shirt, and weighs 1/2 the weight. Unlike the given example of a Darkwood shield, a Darkwood Chain Shirt does not derive any additional benefit, such as reduced armor check penalty, due to its construction from darkwood.