There are three kinds of people who buy gluten-free products: a) People whose doctor has told them celiac disease is what's making them miserable; b) people whose naturopath has told them they're probably gluten-intolerant; c) people whose friends have mentioned they're on a gluten-free diet and now think gluten is really, really bad for you.
Group C, and probably group B, aren't going to be affected by a little gluten slipping into gluten-free products -- which can happen if wheat, rye, or barley are used in other products the company is making. But some members in group A could end up with serious cramps and diarrhea.
According to the Boston Globe, the FDA is coming up with new standards for manufacturers labeling their products "gluten-free." The gluten limit the agency is proposing: 20 parts per million, which is as sensitive as current laboratory tests get.