I didn't know about that, good info. I think ill just leave off the zagg then.The InvisibleShield style skins don't adhere to the glass back of the iPhone 4 as well as they did to the plastic backed 3G/3GS, and in fact, are very easy to pull off the glass. I routinely pull my BestSkinsEver (same material as the Zagg product) on and off my iPhone 4, and just store it on a piece of waxed paper when I'm not using it. The adhesive on the back of the carbon skin would probably bond to the plasticy material of the Zagg, and pull both off very easily.
Thanks everyone else for your input.