Supposedly this happens (not just on the MBP but other aluminum notebooks as well) when a person has a certain ph...the sweat/contact causes pitting. So, what you are seeing, those little dots, is presumably pitting. Honestly, I am one of those people who hasn't rested their hands on the notebook since the Lombard days, where pressure was the problem (wreaked havoc on the trackpad and mouse button). I think it could be prevented by wiping off the areas where your palms rest, with perhaps a damp cloth or some other very mild solution. This would at least prevent further pitting. I think there may also be skins to put over the area, and keyboard, too, but the cheapest way again is to wipe it off.