Any time copyright infringment has gone to court in a clothing-related case, the verdict always comes back that clothing is not subject to copyright, since it is a functional garment (specifically, a "useful article"). Only aspects that have nothing to do with being a garment are subject to copyright law - specifically, a graphic printed on a T-shirt may be copyrighted, but the idea of making a T-shirt in a certain color cannot.
Here's the relevant text from the law
(a) Copyright protection subsists, in accordance with this title, in original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device. Works of authorship include the following categories:
(1) literary works;
(2) musical works, including any accompanying words;
(3) dramatic works, including any accompanying music;
(4) pantomimes and choreographic works;
(5) pictorial, graphic, and sculptural works;
(6) motion pictures and other audiovisual works;
(7) sound recordings; and
(8) architectural works.
(b) In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
I've heard people claim that clothing might fall under (5) as a sculptural work. But that's not true:
the design of a useful article, as defined in this section [defining what falls into section (5)], shall be considered a pictorial, graphic, or sculptural work only if, and only to the extent that, such design incorporates pictorial, graphic, or sculptural features that can be identified separately from, and are capable of existing independently of, the utilitarian aspects of the article.
So, clothing is not copyrightable. What is interesting about the Trovata v. Forever 21 lawsuit that is heading to trial is that they aren't claiming copyright infringement, but rather "trade dress" - that somebody would see a striped shirt and think "oh, that's a Trovata shirt" when it's really Forever 21.
The case doesn't sound terribly strong to me, but I'm not a lawyer. Why should Trovata have a monopoly on shirts with a certain striping pattern? They shouldn't. We'll see what the judge thinks, but this case has no bearing on copyright law.
However, this is a little more worrying (from the same article):
Although U.S. copyright laws do not protect a garment’s basic design, silhouette or form, legislation is pending in Congress — supported by the Council of Fashion Designers of America — to expand copyright laws to the “appearance as a whole of an article” of clothing. The Design Piracy Prohibition Act has stalled in committee. Critics contend its provisions are too sweeping and would stifle competition and commerce in the apparel industry.
Yes, it would stifle competition and commerce. It would also seem to let designers sue home knitters or sewists who make their own clothing that is similar to famous designs. Since the fashion industry as we know it survives on copying the "appearance as a whole of an article," what are those of us who can't afford the original supposed to wear? The wording might mean that changing a few details doesn't get you off the hook, if you haven't changed the "appearance as a whole" - you choose different colors for the stripes of your polo shirt, but it still looks like a multicolored-striped shirt. Can Trovata come after you then? We really, really don't need to create another RIAA-like opportunity, whether it's targeting individuals or other clothing companies.
Then the originality aspect would come into play, I guess. Copyrightable works must be original, which is no problem since I won't accidentally write the exact text of Harry Potter from scratch. But if some big-name designer decides that a red sheath dress with a flared hem is their property ... what happens when I independently decide I'd like to make a red dress and the "appearance as a whole" is similar? What happens to the pattern company that makes a sewing pattern for a sheath dress that can be made in any color?
Clothing is not currently copyrightable under US law. It also shouldn't be. Seriously. That would just be stupid.