Black 8x, on verizon, HW revision 2. The back and sides of my phone are fine. My discoloration is at the bottom corners. At least for the black model, there is a layer of coating that gives it the matte finish, and underneath this layer is the actual shell of the phone, which is also black. I think the discoloration we are seeing is from this layer wearing thin. However, on my phone, I dropped it on its corners once or twice. My layer is not discolored, it is completely peeled away, exposing the smooth black plastic in the corners. Oh, and its also around the USB hub. It shows no signs of stopping, because the edges of the layer are now peeling back.
Real sloppy HTC. You should have caught this within 1 week of testing.
Edit: In short, its not your phone getting dirty, but the outer soft touch layer wearing thin.