I think it is unethical to label something that contains gluten as gluten free just because you can. Instead, I would recommend that the companies work to actually make the product gluten free, rather than uses the FDA thresh hold as a marketing tool and risk making people sick.
Here is my post on how I discovered Rice Dreams contains gluten and is not actually gluten free...and an interesting letter from the company!
It reminds me of the "mercury free" vaccines which are manufactured with mercury and then the mercury is removed. You can't get all of the mercury out-but they still call it "mercury free." People would die if we did this with peanuts or a peanut allergy! If you are sensitive to gluten or mercury your immune system will react to these traces the same as it will react to a large amount. Once you are better or older and your immune system is more developed, you won't be as sensitive, but people who are trying to heal will get sick and they will not know why.
It is also similar to "caffeine-free" coffee which does have traces of caffeine.
Interestingly, in February 2009, Europe changed it's rules for labeling gluten free from 200 ppm to 20 ppm. which is a ten fold reductions in the amount of gluten that the food can contain in order to be considered gluten free.