At Styledash, they mentioned that Nicole Miller is planning on coming out with a new line of clothing for women who wear sizes smaller than "0." It will be called Subzero and it is supposed to be released before then end of the year, although there seems to be no specific date attached yet.
The initial reaction of many to this will be to say that no one should be that thin, but thinness isn't the issue as much as what constitutes a size. In other words, what is now a size "6" may once have been a size "8" or "10" and as the clothing gets larger, smaller women - those who are thin, as well as those who are simply very petite - have a harder time finding clothes.
That being said, is this a trend that will spread to other designers? The US women's sizing system has almost no meaning as it is, and stretching it into negatives doesn't seem like it will help. Switching to a numerical system, based on measurements, might actually help more people find clothes that fit in the long run and clothes that are well-cut are going to serve you better than knowing you are wearing a size -10.