Ive sent the official complaint to TimberTech but I'll post it here also to share with you what I told them. It might be of interest to some of you and I'd like to know if any of you have observed the same whitening characteristics.
In September of 2009 I had a 900+ sq. ft. deck built using XLM Mountain Cedar planks. By early summer of this year whitening of certain planks began to appear. The photos included illustrate the problem and show a pattern which you are possibly already aware.
The deck boards, except where not feasible, are secured with hidden fasteners and, therefore, are grooved boards. As yet, none of these, the vast majority of the planks purchased have not whitened. A number of ungrooved planks were purchased for handrails, steps and certain other locations where hidden fasteners could not be used. Of those ungrooved planks, close inspection shows there are two varieties: one has a more lightly textured surface and the other a more deeply textured surface. As the photos show, the whitening problem, at this time, is occurring only on lightly textured, ungrooved planks and not occurring on any other type of plank.
Both photos show the same railing corner. This demonstrates clearly that problem is not dependent on the amount of sunlight to which the planks are exposed. Photo 1b shows the difference in surface texture.